Get hold of the operator's manual and follow the instructions for resetting it to factory default settings. Then the password should revert to the original default which should be listed in the manual. Then you can change it to whatever you want.
I pushed reset button like it says on back of modem for factory default. That worked but it caused all the computers to stop working. Had no computers for an hour wife finally got it fixed. No options on my computer but she found option on her computer for password. Old password came up we printed it out, typed in the old password now both computers work again.