How do you install a back rim on a bmx bike?

Depends entirely on why it's wobbling, but I'm guessing that the wheel has gone out of true. This can happen either b/c the spokes have gone slack, or b/c of the rim getting damaged in a hard landing or a crash of some kind.

Wheels that have gone out of true b/c of spokes going slack can be trued with good results, but rims that have gotten badly bent by an outside force will rarely become particularly useful again.

Trueing a wheel is something that can be done at home, but it takes time, patience and a bit of skill.

I'll post a link to a site with more info below.