Users are advised to avoid the wireless connection during the Firmware upgrade, it may lead to a sudden disconnection. Prefer a wired connection via Ethernet cable while uploading the firmware in a range extender. Before the firmware update, a user must read the release notes of new firmware to know the configuration settings that might get removed during the process. You can create a backup of those settings and can restore it later using the ap.setup web management window.
https://myap-setup.net/