For everyone that came here by a google search just like I did.
The problem is the order of the command line arguments.
When you use the command like described in the documentation, it failes:
grafana-cli --homepath "c:\Program Files\grafana" admin reset-admin-password mynewpassword
Incorrect Usage. flag provided but not defined: -homepath
But by switching the command line arguments, it works:
grafana-cli admin reset-admin-password --homepath "c:\Program Files\grafana" mynewpassword
t=2019-08-14T11:29:47+0200 lvl=info msg="Connecting to DB" logger=sqlstore dbtype=sqlite3
t=2019-08-14T11:29:47+0200 lvl=info msg="Starting DB migration" logger=migrator
Admin password changed successfully ✔
Grafana Version in use 6.1.3 on Windows 10 Pro (1809).