If you haven't lost the original password and still have access to the host account. Changing the password can be found under the User Profile (click on the users Display Name found to the left of the Login control)

If you lost the host password, and cannot access your portal through your only host account. Your going to have to hack yourself back into the database. Basic steps are as follows.

• Register a new user (do not login as the user, just enter the information in the registration page and click the Register link)
• Access your database through the SQL manager, and open the dnn users table.
• Locate the newly created user, and edit the IsSuperUser column, change the false to true (0 to 1)
• Go back to your DNN site, and login with your newly created hacked host account.

I'm pretty sure those are the correct steps, I had to do this after my 4.8.0 > 5.0.0 upgrade. I thought someone had a guide with these steps, but my google-fu is broken at the moment, hopefully we'll get a stabilization release that fixes my google-fu shortly