Add or change your mobile number in your Facebook account with a computer.
Step 1: Click on the symbol in the upper right corner.
Step2: Select "Settings".
Step 3: Choose "Mobile" to change.
Step 4: Click on "+ Add another mobile phone number" to add your mobile contact.
Step 5: Choose the city you are living and which mobile phone service you are using. "Enter your number" and click "Continue".
Notice: you should provide correct information because it is related to the verification of your identity.
Step 6: Enter the verification code into "Enter confimrmation code" bar then click "Confirm".