How to find previous version of recently changed website

Archive.org may work

The internet archive's wayback machine won't really help you. Having worked in web admin for a govt agency the proper way is to contact the agency under an FOI request. That said I have no idea what the procedure is for Ethiopia.

I should try reading the entire post

I've already tried every possiblity that you've stated.The wayback machine brings very old versions about a year ago.And the website owners are not willing to show me what I wish to see.But I noticed that google has cached pages.Could this help me to get what I needed?

Try it. Keep trying.
It's hard to make something disappear on the internet

In that case, essentially, you are screwed.

As for seeing previous versions of Google cache, I doubt that's possible. There are billions, maybe trillions of web pages cached by Google, and recaching happens at least on a daily basis, if not more frequently. The computing power and expense of saving every single day would be astronomical, even for a moneymaker like Google. I strongly suspect that previous cachings are written over. But hey, you can contact Google and ask 'em.

Hey jes, does google still give access to the cached copies? I haven't seen the links in the serp for years.

It's looking like they don't. I just did a little searching and did not see the listings for cached stuff.