Being under the mount sucks and it’s not a good way to win a match. Here are a few escapes from different variation of the mount.

Low mount:
When your opponent is low in the mount, it’s easier to move them because the are closer to your hips. Here’s what to do if they start to attack:

If they are low and are attacking with a cross collar grip.
then immediately grab their gripping arm at the elbow with your cross arm and pull their elbow out away from your neck. This stops the threat of a choke or neck pressure.
Next, reach up and grab behind their attacking arm with your free arm.
Grab the cloth behind their shoulder or armpit if you can.
Buck them hard up over you as you turn your head facing the side of their gripping arm.
Look up over your shoulder on that side as your roll them over you and end on top in their guard.

Sagging Smother mount:
Sometimes people will mount and then try to psychologically wear on you by sagging on you, which has a smothering effect. Here is what to do in that situation:

If low-mounted and trying to smother you
then frame your arms.
Remember to keep your elbows close to your sides as you block their hips with one of your forearms and put your hands on top of eachother on their hip.
Buck up (Upa) as you push their hips down.
As you come down, put them into your half guard and hip escape away from them.
From here, depending on how much space you’ve create,d you can attack any near arms, or proceed to put them into guard.

High mount:
If someone has the high mount, you can roll them over your body and come out behind them on top.

If they have a high mount
then grab the cloth in their armpits with both arms
Push them up and and extend your arms as you kick your legs up under their arms and hook them one at a time
roll backwards over one shoulder
when their knee is bent, grab a leg around the ankle and guillotine
(this is called the boston crab)
rock back to get the submission