I have those too. Can't wait to replace those cheap things.
1) pull the stand towards the middle of the trailer. You should be able to feel the spring tension.
2)While still pulling towards the middle of the trailer against the spring, pull the stand down. It will swing around to angle away from the middle of the trailer (maybe 30 degrees?).
3) let go and the spring will pull the stand up a little. Also the stand should be locked in this position. If not pull away from middle a little and wiggle - it should lock or it is damaged.
4) the extendable foot has a friction lock. The small lever will release the foot when you push down (I push with my foot when it sticks). Gravity is usually enough to make the foot drop to the ground.
5) release the lever and push up a little to "set" the friction lock
6) optionally you can use a small round rod, inserted through both holes, to crank the foot down. This is similar to a "hi lift" or bumper jack.
As has already been pointed out, you would normally lower the front of the trailer, set the rear stands, then raise the tongue back up. Jim from EggCamper
actually suggested this method. After bending mine a few times trying to jack them up I recommend Jim's direction.
Putting them back up is easy. Kick the release on both stands, hold the release while lifting the foot all the way back up, release the lever, pull the stand against the spring and swing up horizontal and let the spring pull it back into place. A quick wiggle to make sure they are locked and you are done.