You must use your real, verifiable name according to Upwork's Terms of Service.

Go to the Accounts menu > Settings > Click "My Info" to edit your name.

• Changing your name to anything other than a recognized name variation (for example James to Jim) will require ID verification
• To set up payment to your bank account, your name in Upwork must match your name on your bank account
• You can't transfer accounts from one person to another