Actualization

if i install the last version of hikashop waht changes will do in my actual website, i mean the frontendcss and the other changes i´ve made would be changed or not?

If you changed that via the interface we provide in HikaShop you should not loose it.
However, we still recommend that you do a backup with akeeba backup before doing it, just in case...

if i changed several files and save them and afterwards replace the new ones with old ones it would continue working?

I can't say. It depends on the files and the modifications you made. It could break, or it would work.