Not much you can do about it now but stake it. The top is too heavy for the stem and it would probably die if you mounded it with soil. If you had trimmed it down when you first got it you probably would have done it a favor and the roots would have settled in easier.

Use part of the top to take a cutting(s) from; that way it's the momma plant

Basil easily and readily roots in water. Take a couple of cuttings and root them indoors, away from direct sunlight.

The remaining stems will probably put out new leaves at the leaf/stem nodes.

An update: I went ahead and mounded the dirt...I have a nice new basil plant without a long stem :) I just snipped the stem this weekend..it looked like it was turning brown and dieing anyhow. I'm not sure if the old stem will produce new growth or not but I consider the experiment a success as I did end up with a nice bushy short stemmed basil.