to do that you must flash a custom rom since gingerbread is the latest official rom exists.
to flash a custom rom you just follow the guide in general section (procedure is root your phone -> flash a custom recovery -> flash custom rom through recovery)
by overall system what do you mean? you can overclock(increase processor speed and such), by using a custom rom most of the time you'll have more free RAM since the official rom you're using has a lot of processes you don't need running in the background.
pros - much faster phone, you can install stuff that only works on newer versions of OS (such as google chrome and root only applications)
cons - if you choose an unstable rom it might have bugs, either way known bugs should be posted in the thread,
you can brick your phone if you do something wrong (although most of the time fixable but requires much effort from your side)
and you need to learn the process and what you're actually doing.
im no expert but I flashed a few roms on my ace plus.