Login in your online service portal as admin,

Go to Users > User list > Users With Extensions you will get all the User and extensions, then select a user

User Details > Phones & Numbers,

Then you can see Phone tab. You can able to change the phone number from there.

Hope this helps

Hi @Suresh More, you're able to change extension numbers via the path Anirban advised of, but to change an entire phone number you will need to contact support.