First we need to get rid of the old bootloader on the watch and then overwrite the firmware. I assume you haven’t modified your watch in any way. Just the plain, out of the box watch with Weloop Tommy or Chronos Eco firmware. Because that may cause problems.
You will need:
Save those two files as it is (don’t extract the zip) on your phone and start nRF Toolbox. Select DFU and turn your bluetooth on.
Put your watch in OTA Mode. To enter OTA mode just connect a charging cable, press all 3 buttons on the right side of the watch and release top and bottom while keeping middle button pressed 12 seconds more.
Install new bootloader: SELECT FILE -> Application -> select ossw-installer-combined-?.?.bin -> Init packet: No, select device and click UPLOAD. Wait patiently while new bootloader is uploaded.
Install the firmware: SELECT FILE -> Distribution packet (ZIP) -> ossw-firmware-?.?.?.zip, select OSSW device and click UPLOAD
When updating from the original bootloader (first file) the watch will reset every 60 seconds while in OTA mode - you have to start the updating process right after a reset. So it doesn’t while receiving the file. It might take a few tries to get the timing right.
“GATT Error” while updating - Enter OTA mode: To enter OTA mode just connect a charging cable, press all 3 buttons on the right side of the watch and release top and bottom keeping middle button pressed few seconds more.
If you really want to know how things work, check this post out.