With the gun firmly in your hand, use the top edge of your belt, to catch the rear sight, and push down.
To remove the empty magazine, push the magazine release button and flick the empty out.
To insert a full magazine, put the gun in your holster and use the one hand to seat the magazine in, and make sure it clicks in.
You can use the heel of your shoe too. Just kneel down and use the heel, but remember, you will be pointing the gun to the rear, which could be a No-No in range safety.