Hi Jeff,

What you will need to do is reset the modem to factory defaults.  
Carefully insert a paper clip or small object into the reset button and press into it. Hold down the Reset button for at least 10 seconds and release it. The SBG8300 will then reboot, once the LEDs flash the SBG8300 has finished rebooting and will be configured with the default settings.
Afterwards, the default username is admin. The default password is password.

Ok will I have to reset the wifi password and everything  will all the devices disconnect 

Yes, everything will be disconnected at this point. When the modem comes back from the reset (I know it takes an awfully long time) you can use the app to login with the defaults. Note I edited my original reply above and added the default login/password.    Once you get in, you can then change the login/password to anything you want - and please use some method to remember the new password.   

A quick note that may be helpful as an aside, I have been using a technique to remember new logins and passwords as long as I have been working with computers.   You need to do this at the time you are setting up your new login/password. Try to take a screenshot that shows the name of the website, what username you entered and the password.  If the password is just a bunch of *****’s you will need to go, after taking the screenshot, with a paint or similar program that allows you to enter text - and place your new password right by where the password should be.    
When done, save the new screenshot to a special directory that you have setup called “passwords” with a naming convention that you can consistently use in the future.  I use the name of the website, followed by the new password and the extension, jpg, png or whatever it is.  I have found no need to also include the username because nine times out of ten it is my email address.  By using the name of the website first, and as you accumulate more password screenshots, you are basically setting up a simple database of passwords for future use.  My ‘simple database’ now has more than 650 entries in it and if I have to find a password, I just look in the password directory, page down until I find what I’m looking for (remember filenames are always listed alphabetically) , and usually don’t even need to load the stored image because the password I’m looking for is right there in the file name.  

Hope you find this tip useful.