The system updater provides OTA (over the air) updates of /e/OS version at approximately monthly intervals.
The system updater is usually the best way to go. If you have the correct TWRP installed it is expected to be compatible with OTA updater.
Android version Upgrades usually have to be done manually. A little more insight here e/OS + Android OS version names and numbers? - #3 by aibd.
If you are in a different situation it would help if you provide info in this form [HOWTO] Give complete /e/OS version info easily for support, answers, comparison etc of what you are running.
There is an Upgrade page for your device https://doc.e.foundation/devices/river/upgrade anticipating the soon to be published Android T version