Here’s how you do it.
Login in into your account from the laptop or desktop version on the mobile devices (like from chrome/safari/ etc) and then click on your picture which opens up the tab where we can see - profile, account, invite friends, etc.
Click on the profile, it shall open the profile page.
Scroll down till you see the old number and then simply add a new number and verify the number using phone call or sms and once it has been added to the account you will be able to delete /remove the old number from the account.
Hope me it works and enjoy your vacations. 😊