More and more Deco products gradually support "Auto Update". It allows your Deco to automatically update to the latest firmware which provides better network performance. Occasionally users might want to roll back to a specific version.  This post will guide you to properly downgrade Deco firmware.
If you need to disable "Auto Update", please go to Deco APP>More>System>Update Deco and disable the "Auto Update".

1. Find the suitable firmware version under "Download Center" for a specific Deco model.

Model and hardware version availability varies by region. Please refer to your TP-Link regional website to determine product availability.
Vx.0=Vx.6/Vx.8/Vx.9(eg:V1.0=V1.6/V1.8V1.9)
Vx.x0=Vx.x6/Vx.x8/Vx.x9 (eg:V1.20=V1.26/V1.28/1.29)
Vx.30=Vx.32/Vx.33 (eg:V3.30=V3.32/V3.33)

2. Downgrade the firmware via the Deco Web UI or Firmware Recovery Tool.

Case One via Deco Web UI

If you want to downgrade Deco M5 from 1.7.8 to 1.7.6, these two versions are minor firmware optimization and could be downgraded directly through web UI>Local Upgrade.
-Deco web interface login

Case Two via Firmware Recovery Tool

If you want to downgrade Deco M5 from 1.7.x to 1.6.x, these two versions have crossed different firmware platforms and need to be downgraded via Firmware Recovery.
Firmware recovery will reset the existing network configurations. If you don't want to downgrade the firmware and ask for other suggestions, please start a new thread here.
Deco Firmware Recovery tool only accepts firmware bin files less than 32MB. If both firmware versions are over 32MB, you might need to recover to the middle version first, and then to the target firmware. Such as downgrading Deco X50_V1_1.5.5 to 1.4.3, the users need to

• Perform a firmware recovery on one of the satellites Deco X50 first.
• Try to upload an older official version, 1.2.2_Build_230301.zip for example, 
• Then update to 1.4.1 from 1.2.2 via web UI.
• If this process works for the satellite Deco X50, Please repeat it on the main Deco later.