A registered mobile number with Gmail provides a few security benefits. Along with allowing you to recover your account in the case of lost password, you can also use it to login without a password.
But, if you want to change the registered mobile number on your Gmail account, you can use this procedure to do that.
Change Registered Mobile Number in Gmail
Step #1: Open Personal info & Privacy page for your Google Account. This will take you to the page where you can decide what you want Google to know about you. Scroll down a little and look for the Your personal info section.