One of the salient virtues of the semi-automatic pistol as a fighting weapon is the speed with which it can be reloaded. The technique of making a speedy reload is pretty simple—dump the magazine that's in the gun by pushing the magazine catch and letting it fall free. Then insert a pre-loaded spare and slam it home.
While that is the essentials of the technique, good schools teach more. First, try not to let the gun run dry and go to slide lock. Doing so will require an extra hand motion to get back in the fight. When you decide to reload, get a positive grip on the replacement magazine—with the trigger finger of the support hand running down the front face of the magazine.
Only when the grip on the spare is positive do you use the thumb of the shooting hand to push the catch and drop the partially expended magazine on the ground.
Position the rear flat of the spare to the rear flat of the magazine well and drive it home with the heel of the support hand. A good pistolero can perform a speed magazine change well under a second. It is a basic technique that must be mastered.