1.To change your contact details in gmail account, first of all sign in to your gmail account.
2.Go to the settings. Select the account setting option .
3.In that select other advance setting option.
4.There you will select personal data setting.
5.Now you can see an option to change your mobile number in profile setting.
6.Click the edit option to change your mobile number.
7.As soon as you enter another mobile number, you will get a code on your new mobile number to verify the same number.
8.Use the code and update your mobile number easily.