Please follow carefully thease instructions
The process might be intimidating, but it is quite easy.
You can't damage your device, if you fail to proceed, just start over.
More detailed step-by-step instruction: EN, DE (by Kilian Epp)

1. For Windows only: Configure driver (only for first time)
2. We recomend to use newest Chrome or Edge for this utility
3. Connect Strato data port to PC
4. Connect Strato power port to charger or PC
5. Click Connect Strato
6. Hold both arrows and menu button for 10 seconds
7. Then relase one arrow
8. When the device appears on the list release all buttons on Strato
9. On the list select "DFU in FS mode", Click Connect
10. Click Update bootloader
11. Wait for update to finish