Upgrading Android version on phone

I've got a Galaxy S2 with Ice Cream Sandwich on it (4.1.2) and want to bring it up to Lollipop (5.x) in order that I can install some apps I want, because they're complaining it's not supported.
There are youtube vids on this but I wondered if these were the best way to go??

Looking at this:


and this:


Any recommendations on how to back up stuff? Is this an SD card job?

.....next challenge will be a Galaxy Ace 2 from Gingerbread to Lollipop - it'll probably catch fire!