Yes, it's possible. Use the Tempo Global Track:
Click on the Global Tracks triangle above the Track List in the Arrange area.
Right-click on the Global Tracks to choose a "Global track to view". Here you can change the tempo of your project over time.
Is that what you wanted to achieve?
If all you need do is adjust the tempo of the loop. e.g. make the loop a faster or slower tempo but keep the project tempo constant there are a few ways to do this.
Check out the Section of Tempo and timestretching in the Logic 101.