Within just a few seconds, you can remove and replace the battery in LG's G5 thanks to its new modular design. What's a modular design? I'm glad you asked. In short: Through this design, the bottom of the phone slides out, bringing the battery with it. Once it's moved, you can then replace it with a fully charged battery or another of the LG's accessories designed to interface with the phone. There's a camera module for better control over the camera app, or a speaker with improved audio features.
To remove the battery, locate the physical button on the bottom left corner of the device. Press and hold the button in, then pull the bottom of the device down.
The button on a unit I've used for testing was fairly hard to press -- which is a good thing -- so if the battery isn't easily coming out, press harder on the button.
With the battery removed, you now need to disconnect the battery itself from the bottom portion of the phone. You will likely have to use more force than you think you should to detach the battery, or at least that's been my experience.
I found it easiest to hold the bottom of the phone in one hand, while pushing the top of the battery to the left with my other hand.
Once the battery comes out, simply reverse the process to put your second battery into the phone, making sure to line up the arrows on the battery and the bottom piece of the phone.