There is sadly not built in way to easily do this.
The below instructions should work for sqlite3. If you use MySQL or Postgres the SQL command should also work.
$ sudo sqlite3 /var/lib/grafana/grafana.db
sqlite> update user set password = '59acf18b94d7eb0694c61e60ce44c110c7a683ac6a8f09580d626f90f4a242000746579358d77dd9e570e83fa24faa88a8a6', salt = 'F3FAxVm33R' where login = 'admin';
The above command sets the password for the username
admin. If you changed the username for your admin user you need to modify the command accordingly. Since the passwords are hashed in the database it looks like the password is set to just random text.