You have two choices:

1 - Call and ask UA to convert your ticket to a one-way and issue FFC for the residual amount.
2 - Call and add a new return flight that is combinable with your original outbound. You'll receive FFC, or pay a fare difference, depending upon what the new flight costs.

You will not be able to treat this like the other FFC types where you can apply it during booking until you do either #1 or #2.

Note that if your original fare was a round-trip fare, #1 may leave you witth very little leftover credit -- potentially zero. This is particularly true on international fares.