Well it's is a pre-built PC. Often OEMs like Dell and HP lock down their BIOS on a given build, meaning you can't upgrade the CPU. However with his being an ASUS pre-built desktop, likely it won't be a problem. And there is a significant performance difference between the i5 6400 and i7 6700 CPU in apps that use hyperthreading. The 700MHz increase speed of the i7 will help games at 1080p, but not by a significant amount.
The only way to know for sure is to get on ASUS's customer support/forum page and ask. While you are at it, you may as well upgrade to 16GB RAM as well. My guess is that it came with 8GB which these days is minimum gaming requirements with 16GB recommended.
Regarding installation, just search on Youtube for videos showing your PC or one close to it in design by ASUS being upgraded.