You need to ROOT your device first.
This link helps you to ROOT and install custom Recovery on your device.
You can download custom ROM from here.
After rooting and installing custom Recovery(TWRP), Download ROM file zip,GApps zip(if you need to install Google apps), .
Reboot to recovery mode.
Prepare a backup of current ROM (IMPORTANT). In TWRP, select backup, and check System, Data, Boot, and select backup.
After that, go back and select Wipe, select System, Data, Boot, Caches. Wiping deletes all your data.
Then, go back and select Install, and select zip file.
And you can install Gapps, select Install, and select zip file.