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Flash firmware with nRF Connect APP
Download nRF connect APP
Download Firmware files to your Phone from "10 Minutes Sleep Firmware" Tab
Turn keyboard off, press and HOLD 4 keys then turn keyboard on.
Open nRF Connect App, will see KCDFU in the Scanner tab, then connect it.
Click "Open Document Picker", select the file you downloaded earlier (Which Keyboard/side you have?) then click "Start".
Waiting for the progress bar to reach 100%. Done! Do step 1-4 on the other side.