Well we don't normally allow that but since you asked I thought of a way to do it but ....ssshhhh....don't tell anyone else.

1) Delete the .fseq file.
2) Edit the XML file for the sequence and in the header where it says "50 ms" change it to "25 ms".
3) Re-open the sequence in xLights and there ya go.
The only issue I can think of is if you have any Data Layers created they won't be the right size so you should delete them before you do this and then Import them again from scratch.