Just push more commits on to the branch the request is for. The pull request will pick this up then.

Example:

If you want to have b merged into master

1. You push c1,c2,c3 to b
2. then you make a new request for b
3. it gets reviewed and you need more commits
4. You push c11,c21,c31 to b
5. The pull request now shows all 6 six commits