Since I “loved” @fzinken solution, but thought, lets make it an option to have after a change. Like I do forget passwords on a regular basis. I did a small Kodi 18.x(apparently works on v.19, right now) addon, which you can install into Kodi, and when ever you need it it should be under “Programs” in main menu.
download this zip file:
Passwd_Restore
Transfer it to your OSMC device, open Settings->Addon browser->Install from zip
Chose homefolder and navigate to “main.zip” or if you renamed it, to the filename you chose.
What this addon does is restore osmc user to the default password: osmc