there are a few ways to do it.

You have to click on the channel settings of the channel that you have the sample on. There is a tab called time stretching. There are three knobs 1)pitch, 2) mul, and 3) time. This way works best if you know the tempo and how long your original sample is. For example, if I import a 4 bar sample that's at 76 bpm into an audio channel. I will set the global tempo of the project to 76 bpm then I will set the time knob to 4 bars. You can turn it manually or you can right click it and select 4 bars. You can now change the tempo or the pitch and it will remain a 4 bar sample . You have to practice with it until you get the hang of it because it's tricky. When you change the tempo, either the pitch or the length of the sample has to change. In order to keep it a 4 bar sample at 76 bpm fl time stretches for you.

I hope this helps, I'm not the best at explaining this. If you don't get the hang of it at first don't give up. I had to mess with it before I understood.

This is the way I typically do it. I rarely change the pitch though. And I don't use the mul knob (multiply knob), at least not yet.