Increased or have increased?

Either is fine. English speakers often uses the present perfect to make past actions have heightened importance or to be more formal, which shows a type of heightened importance.

I've won the lottery!!!
I won the lottery.

Both forms sound correct to me, but psychologically, increased is direct and assertive while have increased implies that it might not always happen. The less information you provide about the past of your company growth if uncertain and fluctuating the better.

Neither of your ideas expressed above have any merit as regards how English works, Fil.

You know I am not a native speaker, not even a super fluent one. My input was intuitive and probably formally wrong. But then again maybe not...
"Have increased" means the process is in flux still.

I know you are not a native speaker and I was bad mouthing you, Fil.

That is ONE of the uses of the present perfect in English. As in,

"I have lived in Portugal for X years." which entail the speaker still resides in P.

"I have just eaten." means the meal is over/finished/done.

Oooooopppppsssss, missed a "not", Fil.

and I was NOT bad mouthing you, Fil.

Yes and then again it is surely less assertive. I think we can agree on that, can't we?

Should I use the simple past or the present perfect in the sentence below?

Over the past year, our company's sales increased/have increased by 85%.

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To agree with you would be to deny the reality that is English, Fil. Using have + past participle is actually a way for English speakers to be more assertive, more emphatic, more in your face, more of these same types of situations.

I have had enough of you! Get out. [just an example]

She has won the lottery!

I've got/gotten a miserable cold.

Over the past year, our company's sales have increased by 85%.
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This have/has + PP is often used to identify hot topics/highlight important past time events [The president has been assassinated!]

Thank you for clarifying. In Portuguese, it works the other way around.
At first inspection it seemed to work with English too, in many cases it doesn't I know, but in this case, it made somewhat sense to me.

Interesting. Might I ask, did you learn English in your school system, from a language school, ... ?

Yes, I did but not from a language school, it was in the public one as a third language after French as a second...I forgot a great deal of French over the years but my French accent is still great...
...now seriously my English improved dramatically when I lived in London for around 1 and a half years. Still is far, very far from being good. I'd like to think I have improved a little bit over the years and this forum along with other online places have helped me on the way.

I HAVE forgotTEN a great deal of French over the years but my French accent is still great...
...now seriously my English HAS improved dramatically [when I lived in London for around 1 and a half years. ]

I have edited some of your comments ONLY to highlight how a native English speaker may also present what you have written/you wrote.

Notice the brackets around your time adjunct "when I lived in ... ". Using the PP + past participle is almost never used, closely connected, with a time adjunct. This is because the purpose of the PP + past participle is to highlight a HOT TOPIC and recognizing that it is for that purpose only ELNs don't connect it to past time adjuncts like yesterday, last week, when I lived in ... .

...yes forgotten of course. Once I hear it read it I recognize the mistake...and yet, I repeat the same mistakes all over again the very next day.

I wasn't pointing out any mistake of yours, Fil, I was trying to show you how a native English speaker might use the PP+PP where you have used the simple past.

This particular use of the present perfect is the hardest one for non native speakers to grasp and put into use. It mostly used in speech tho' it does see use in headlines to describe HOT TOPICS.