to go uptown on us?

You need to provide some context. The meaning of the expression depends upon the context. For example, if one were playing a card game which involved a trump suit, it would mean being obliged to play a high-ranking trump card in order to be sure of taking a trick. Most of that explanation might be obscure, so without context, there is no way anyone can explain the meaning to you.

To provide context, please post the sentence in which you read the expression.

The context is when the son of the family won an award, and he was proud of it. But his father said:Nobody likes a bragger. What are you? going uptown on us?

The father's command of English is poor, then (or whoever wrote it has a poor command of the language). The word in English is braggart, not bragger. In this case, the use of the expression is obscure, but it seems as though the father is suggesting that the son is attempting to seem more important or accomplished than actually happens to be the case.

However, i've never seen this use of that expression.

So what you mean is when we talk about a person who brags about himself, we use braggart, not bragger?

I'd say I am not good at English. And I am sorry for all the trouble I've caused you.

And I'd say the speaker is probably Irish American from New York City.
brag [bræg]
vb brags, bragging, bragged
to speak of (one's own achievements, possessions, etc.) arrogantly and boastfully
n
1. boastful talk or behaviour, or an instance of this
2. something boasted of his brag was his new car
3. a braggart; boaster
4. (Group Games / Card Games) a card game: an old form of poker
[of unknown origin]
bragger n
bragging n & adj
braggingly adv

Bragger is an American (and perhaps elsewhere) version of braggart. It's a good solid word. Braggart isn't used very much except in more formal circumstances which leads me to "uptown".

To go "uptown" is to act if as you were better than you actually are. (It may come from the days when the poorer neighborhoods were in the Lower East Side and the upper classes lived in the Upper East Side.) Uptown people are richer, better educated, better dressed; downtown people are not.
See this video about a guy who definitely wants to have a part of the uptown life. http://www.youtube.com/watch?v=2F-nt7aC_JQ

Joe(downtown guy)Nation
PS: Is is not necessary for you to apologize to any of us here. We are here to help.

Nonsense . . . it's no trouble at all, and we all relish the opportunity to display how erudite and clever we are, or wish people to believe we are . . .

I am not disputing what Joe has to say. I have never in my life encountered the word "bragger." By the way, although i am an American of Irish descent, and was born in the city of New York, i was not raised there, and i will not stoop to using their silly patois.

(Put that in your pipe and smoke it, Joe!)

If you want an example of the use of "uptown" to imply high class or wealthy, look up the lyrics to the Billy Joel song "Uptown Girl".

Excerpt

Right, engineer.

Joe(Hey, what's this stuff in my pipe?)Nation

Now I've had the opportunity to view it twice, I really, really like the girl in the video.

Explain to the guy about the numbering system of the streets in New York/ American cities, and the relative affluence/ attractiveness of the suburbs.

Now to go "downtown", on the other hand:

When you're alone
And life is making you lonely,
You can always go downtown
When you've got worries,
All the noise and the hurry
Seems to help, I know, downtown

Just listen to the music of the traffic in the city
Linger on the sidewalk where the neon signs are pretty
How can you lose?

The lights are much brighter there
You can forget all your troubles, forget all your cares and go
Downtown, things'll be great when you're
Downtown, no finer place for sure,
Downtown, everything's waiting for you
(Downtown)

Don't hang around
And let your problems surround you
There are movie shows downtown
Maybe you know
Some little places to go to
Where they never close downtown

Just listen to the rhythm of a gentle bossanova
You'll be dancing with 'em too before the night is over
Happy again

The lights are much brighter there
You can forget all your troubles, forget all your cares and go
Downtown where all the lights are bright,
Downtown, waiting for you tonight,
Downtown, you're gonna be alright now
(Downtown downtown)

Downtown
(Downtown)

And you may find somebody kind to help and understand you
Someone who is just like you and needs a gentle hand to
Guide them along

So, maybe I'll see you there
We can forget all our troubles, forget all our cares and go
Downtown, things'll be great when you're
Downtown, don't wait a minute more,
Downtown, everything's waiting for you

Downtown (downtown) downtown (downtown)
Downtown (downtown) downtown (downtown)
(repeat and fade out)

I didn't mean you personally , Joe.

In the old b & w cop films and tv series, the suspect was always being "taken downtown" by the arresting officer. To headquarters. The centre, sorry center, of the city.