How to factory reset a Bryton Rider 15 bike computer?

A friend bought one of these used and it looks like its in really good condition BUT it still has all the original owners rides logged. He wants to use it for Strava and there seems to be no way to remove the data from the head unit - as soon as it's connected to his Bryton account it wants to upload ALL the old rides and will do the same once it's linked to Strava.

I can't find any info on how to do this online and definitely nothing in the manual. The only reference to reset is a soft reset, ie restarting the device if it's frozen. I saw something for another Rider model saying to press all 3 buttons and he's tried pressing and also holding, both with it turned on and off.

He's emailed Bryton and contacted them through facebook but hasn't had any replies at all. (their Twitter account is dead in the water now)

Any suggestions?