Company Account

First, you'll need to recover the password to your company's account.  To do so, go here.  Once you get to that page, you can enter your company's username (or the email address) associated with the account.  Twitter will then email you the password.

Once you get the password, just log in to Twitter using your company's username and the password you just received.  After you access your account, you can start making all the changes you want.  You can get help on changing your username.  And, if you want more personalizations,  here is a good way to start!