I have a pair of TP-Link Deco X90 devices. They are routers, but I am using them as Access Points. I upgrade their firmware yesterday. Here are my procedures.
First, double-check the hardware version of them and the country.
Mine is the US edition, the hardware version is 1.0
The latest firmware is 1.2.6. Download it and unzip the file. The firmware file is .bin.
Sign in and go to the Advanced page, System, Firmware Upgrade.
Choose Device Model X90, Browse the bin firmware file, and click UPGRADE.
It will upload the bin file and upgrade the devices.
The new 1.26. firmware has some improvements and bug fixes:
1. Enhanced the security of the device.
2. Optimized the time limit settings of parental control.