When you bought a second hand blackberry smartphone or someone has given you his old blackberry to you, then you should remove the previous owner’s blackberry id and change it with your own bb id.
Unfortunately, for some models you cannot sign out from the current blackberry id and sign in with different bb ID without a security wipe, so heres how you should first wipe your bb smartphone to a factory setting default so that you can remove the previous owner's bb id and sign in with your own.
Go to Options from the home screen > Security > Security Wipe > you can check all to wipe all the data in your blackberry smartphone and everything would be set to factory setting > type “blackberry” to confirm the wipe > then click Wipe Data.
With the above steps, all of your contacts, emails, media, applications etc would be wiped off, and after that you can sign in with your account.
If you forget or want to create a new blackberry id, then you can go to http://us.blackberry.com/bbid.html > click the link to create your new blackberry id > click I Agree > complete the required fields in the sign-up page > click Save. After this step, you should receive a confirmation email, click the link within the email message (the link will expire after a 24-hour period), when prompted, enter your blackberry id and password.
Once your blackberry id has been created, you can anytime change it by logging into the Blackberry ID website and then edit your account details:
Login to your Blackberry ID website with your username and password > click Account Details > Edit > replace the current email address/username with the new ID > Save > Done.
To change your Blackberry ID username on Blackberry Bold 9900 4G:
Go to Options from your home screen > scroll to select Device > Blackberry ID > scroll to select Screen Name > after that delete the existing username and enter the required username > now press the Menu key > Save > Enter your Blackberry ID password > Ok.
How to find out what model your Blackberry is:
Press Menu button from your smartphone > select Options > select About. On some models you have to select Device and then About/About Device Versions. Now you can see the model of your Blackberry and its software version.
Other quick method is to go to Message folder from your home screen, open up your email and type “Myver” on the Subject empty field and then press space button on your phone. After that the model of your blackberry smartphone as well as the OS version will appear immediately inside the blank field.