Update from 3.3.1 to latest

Hi there

I haven’t entered for a while (since 3 years I guess) in my WordPress panel, only to realize that obviously it’s really outdate.
But even worse, I can’t access the panel, most functions are not available. I only see the left menu… which means I can’t upgrade right from the panel.

Do you think just uploading the unzipped WordPress.zip to my FTP (right where I have it right now) would be enough to have everything working again? Would that erase anything I have (posts …) ? Or is there a way (like a url I can execute once logged to my panel) to direclty trigger the update?

Thanks a lot in advance!