Having trouble changing to a different router after moving, from Suddenlink service to Spectrum.

Model EX6400  

My son helped set this up initially, so I do not know what he did. 

I purchased this product several years ago.

I have moved to a different state and service from Suddenlink to Spectrum.

I cannot get the Suddenlink passwords removed to change to my new Spectrum password.

Can anyone tell me how to do this?  I am pretty ignorant of this sort of thing so I need very

detailed steps to do so. 

Thanks to you in advance if you can help me.

Jim R.