You can't update a rooted phone (depending on what was done while it was rooted, an update could turn the phone into an expensive paperweight), but only a very few apps won't install to rooted phones, like some banking apps, for security reasons. (A rooted phone is considered a security risk - and it is if you don't fully understand what you're doing.) Are you sure you mean "rooted"? That's something that doesn't just happen (it's almost impossible to root a phone, which is why some phones take many months until someone figures out an exploit to use to root the particular phone), it's something you did yourself - deliberately.
If that's what happened, that you rooted the phone, back up
everything, (
Backing up an Android Device) then reflash the stock ROM (
[Samsung] How to flash Stock ROM via ODIN). Then see if you can find a rooting method for that Grand 2 running that new version of Android. then restore from your backup.
If not,
register an account here so you can continue the conversation here, instead of starting a new one every time you post., and let us know exactly what happened to put the phone into the state it's in that doesn't allow it to install apps.