If your mobile phone number has already been added to your account you will see a Delete my phone link under the Save button (shown below.)
Click the Delete my phone link and the mobile number will be removed from your Twitter account.
After you have removed your mobile number, the screen will give you the option to connect a new mobile number to your account. Follow these instructions if you'd like to add a new mobile number.
To remove your mobile number via SMS:
Using the phone you want to remove from your Twitter account, send a text message containing only the word STOP to your country's Twitter short code).
Your mobile number will be removed from your Twitter account immediately. To add a new mobile number via SMS, follow the instructions listed here.
Having trouble?
Our troubleshooting article has answers to common problems with adding or removing mobile numbers.