Assuming your new mobile is replacing an old mobile with Okta Verify, first you need to reset (aka clear or delete) your old Okta Verify relationship with your old device. Click on your name, then Settings, scroll down to the Extra Verification section and locate your Okta Verify factor enrollment.

Once you have enrolled in any MFA factor, Okta increases the security of your account by prompting you for MFA before it allows you to modify anything on your account, including your MFA factors. Unfortunately, if you only enroll in one factor type (like Okta Verify) and then replace the device from you enrolled, you will be unable to remove the old enrollment. Your only option in this situation is to contact your Okta administrator, who will have to remove that factor enrollment for you.