You just need to add another milestone. Perhaps this will help:
ETA: You need to release the first milestone before you can set up another one. I've flagged your post so hopefully a mod will come along with more info.
Or ... what Jennifer D. suggests. Her advice is always spot on.
David, when you initially create a job you can deposit the whole budget into escrow or just choose to deposit enough for the first milestone (which is what I assume you did - this is what I usually do too.) If you only deposit the funds for the first milestone, then after that you can only fund one milestone at a time, which is annoying. So you could either:
1) Release the first milestone you created, then create a second milestone for the remaining budget (not really recommended if you don't have the work yet, because you won't be able to get it back later)
2) Pay a small token amount on the first milestone (like $5), which will leave the remaining funds in escrow, then set up a second milestone for the remaining budget minus whatever is left in escrow from the first milestone
3) Set up a *new* contract with the entire amount funded (including whatever you have in escrow for the first uncompleted milestone), then close the first contract and request the funds currently in escrow be refunded
In any case, you should discuss this with your freelancer so they know what is going on. Also remember that you can set up a milestone for as much as you need, then pay a smaller amount and then set another milestone - you can keep paying less for each milestone and setting a new one for as long as you have money in escrow, without paying anything extra.