1. Find New Magazine
When you have engaged a threat and partially expended your magazine, you should try to avoid running out of rounds. You do that by changing out your partially expended mag with a new full mag whenever you have a chance to do so. This is called a Tactical reload.
You start by finding a full mag from your reloading source when you get a chance to do so.
2. L-Shape Grip
Now place the new mag on the side of the expended mag according to the picture above. You will then be able to grab the bottom of the expended mag and hold on to it before you eject it from the mag well.
You want to hold on to the expended mag so that you can save the ammunition for re-distributing into one full mag. Or for use as is, if you run out of full magazines before you run out of enemies.
3. Turn 'n' Insert New
Now eject and pull the partially expended mag from the mag well and immediately turn the two magazines so that the expended mag moves towards yourself.
This will turn the full mag to the mag well opening, where you will then insert the new mag. Push-Pull and let go of the inserted mag but hold on to the expended one.
4. Save for later
Last step is to put the partially expended magazine back in your kit. Depending on urgency, you should either dump it in a dump-bag or place it in a mag pouch of your choosing.
Things to consider when placing partially expended mag:
Place it so you remember where it is and so that it will be the last mag in your order of mags.
Magazines in a dump-bag will make more noise than if placed in a mag pouch.