How to update TWRP from within TWRP? I'm in TWRP after a factory reset and I'd like to avoid to install some OS to update TWRP with the [TWRP update app TWRP from within TWRP?

I have a Moto G2014 and previously had Cyanogenmod 13 installed.
Downloading coming from the list of devices allows to download a .img image which can be transferred via MTP-USB connection and installed after selecting Images in the Install file browser. When asked to select between boot and recovery partition select the recovery partition (selecting the boot partition has at least no effect and might be harmful).

It's possible to flash a newer version via fastboot as well.