Update my cms from 2.3.5 to 3.3

I want to update my cms from 2.3.5 to 3.3 but I am really afraid that it will not go well.
I am hosted in a web server
and so my logic tells me to copy via filezilla all the content of the new cms and replace it with the old one by just keeping the library and recpier the file setting.php
is this the precedure to follow and in the worst case, can reversing the old version
thank you