If you have forgotten the root password or if the root password no longer works, you have to use the Failsafe Mode and Factory Reset.
From there, you don't have to reset the whole configuration. Note that fail safe mode does not require a password for authentication of root (!)
You only have to mount the root file system and set a new password with
passwd and then trigger a restart. In fail safe mode,
passwd will not ask for the old password (that you may have forgotten):
root@(none):~# mount_root switching to jffs2 overlay root@(none):/rom/root# passwd Changing password for root New password: Retype password: passwd: password for root changed by root root@(none):/rom/root# reboot -f