Make a new difficulty.
Go to timing.
Choose a place to put the slider velocity (in this case you'd probably it right on top of what determines the BPM.)
Create a new timing section.
Check the "Inherit previous timing settings" box.
On the side, you'll be able to choose things like 0.75, 1, 1.25 as well as a custom time.
Select one or input your own time.
Save.
Simple as that.