Learning the reload drill for the AR-15 with this expert is something you are going to want to learn for tactical applications.
Steve Horsman demos the reload drill for the AR-15 with the new Saint AR-15 from Springfield Armory. You are going to want to practise this again and again to become proficient.
A good drill to practice so you can build muscle memory and do it under high stress situations. Do it the same way each and every time and have your gear laid out exactly as you would wear it in a tactical situation so that you know where each and every magazine is.
The only recommendation I would add is to check the position of your bolt on the AR before bringing it into your workspace to deal with the reload. This was hammered into me in formal training, so that you take a nano-second to know the position of your bolt visually. While your mag may be empty, it could also be a malfunction, such as a hung-up casing that blocks the bolt.
Something like that would require another immediate action rather than a mag change and likely a transition to your secondary gun, a pistol, depending on distance to the threat and situation. That is a whole other drill that will be featured later but food for thought.
If you own an AR-15, be sure to get out and go through these drills dry and live, better yet with professional instruction. It is a skill you will want to know intimately for future situations.