You can link your mobile phone number in settings: https://vk.com/settings.
To change a phone number that is not available now and link a new one, you should wait for 14 days. The number will be changed then.
If you don’t want to wait for 2 weeks, you may fulfill this form: https://vk.com/restore?act=return_page.
Even if following the instructions you cannot link your number, write us about it. Your request will be reviewed in a shorter time if you write us the phone number and a link to the page. Thank you!