Iphone 4 upgrading from IOS 6.1.3 to IOS 7.1.2, is it worth it?

My mom got an old Iphone 4(24GB) which she's been using for like years~ Currently the iphone version of hers is 6.1.3 and I wanted to upgrade her phone to 7.1.2 because some of the apps and features are not available in the current version.
1st question
Is it worth it to upgrade? Will it be laggy or buggy than current version?
2nd question
Should I backup before upgrading?
3rd question
I saw some forums they metnion about SHSH Blops, whats that? They said we should backup that so If you doesn't like the new version, you can revert it back to the old version. Then again, should I backup the data before downgrading?
4th question
How to backup stuff in Iphone and restore it back?

FYI, I am not an apple user so I have no idea what is it about in that phone or whatsoever, I'm pretty sure my knowledge is above average when comes to android.

Thank you! Your answers will be much appreciated.

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