I have a new mobile phone number and cannot remove / delete my old phone numbet as iMessage phone numbet. Can you help me solve that problem?

See if anything here helps.

FaceTime and iMessage - Link your phone number and Apple ID for use with

iMessage on your iPhone or online - Deregister

If you are signed out of Messages/FaceTime and can't remove the number,  contact Apple ID Support.

Apple ID Support

Apple ID Support (2) 

Apple Support by e-mail or chat    Work through the screens to get a chat option.  You may need to pick a problem that is slightly different than yours to get a chat option.  

Apple Support using Twitter

Or fill out and submit this form.          The form is for iTunes, but it will get passed to the ID Support section.  

Sign out of Messages/FaceTime. Log into your Apple ID and remove the phone number. Wait a while for the change to sync. Sign into Messages/FaceTime.

Apple ID - Manage

You are welcome. Have you tried adding the new number to your ID to see if you can then remove the old number. After adding the new number, you may need to log out of the link and back in.