There are a few different ways to install a new Android system. The most common way is to use a custom recovery like TWRP. With TWRP, you can backup your current system, wipe your data, and install a new Android system.
Another way to install a new Android system is by using a tool like Fastboot. Fastboot can be used to install a new system image on your device.
Finally, you can also use ADB to sideload a new Android system image onto your device. Sideloading is a bit more complicated than using TWRP or Fastboot, but it can be used if you don't have a custom recovery or Fastboot installed.