Click in the "Time Signature Numerator" box at the top left of Ableton. It is the fifth box from the left, reading "4 / 4."
Click and hold on the left "4" and drag your cursor up or down to raise or lower the value. You can also click to highlight the number and type in a new number.
Repeat Step 2 on the right "4."
Right-click on a scene (any of the numbered boxes under the "Master" column on the right side of Ableton) to rename it.
Type the desired time signature in "number/number" format.
Hit "Enter" or "Return" on your keyboard when you have finished renaming the scene.
Repeat Steps 1 to 3 for multiple scenes.
Click "Play" (the sideways triangle) in the scene you want to use first. Doing so changes the master time signature. You can continue to change the time signature by playing different renamed scenes.