TWRP updates are easy. Two ways, wait to be informed or check for updates using the updater app. For this way is called ota updates and once you have been informed or found it from the app, you can select to download the update. When ready, it will ask you if you want to install it and warn you that your system is encrypted and that when rebooted in to twrp, you will need to enter your screen lock password. Hit OK and do as it says, input your password and it will do the update, wipe dalvik and cache as part of the script and may ask or simply reboot in to system. First boot will be slower as the cache has been deleted.
Manual updates are nearly the same. Find and download any update, reboot to TWRP, input your password, select install, select downloaded ROM, swipe right to flash. Once done, select wipe cache and dalvik, swipe right and once done select reboot system.
