How can I change the length of PRF output?
I don't understand why you are talking about a 1-1 property; a pseudorandom function is not 1-1. As such, you can always just truncate in order to reduce the output size, and you can just truncate by taking the first $t$ bits that you want (you don't need to apply any other function). In order to extend, if $\ell$ is long enough to be the length of a seed in a pseudorandom generator, then you can apply the PRG to the output of the PRF to get a longer output.

If you really are talking about a 1-1 function, then you want to change the output length of a pseudorandom permutation. This is much more difficult, and can be done. A place to look is at the whole body of work on "format preserving encryption", which is problematic in name (it's deterministic so not secure encryption). However, what they really do is construct permutations over domains of arbitrary size.