You can only change your mobile number if your existing number is not authenticated.
You Can do that by posting an ad, and on the last step you will be prompted to authenticate your number.
Click on the x sign next to your old number.
After that you can change your number and click on change.
If your number was already authenticated then only admins can modify that information on your account.
For further assistance, we kindly request you to contact our customer support team by visiting http://dubai.dubizzle.com/contact/