To Update a field in the customer record from registration page of a site builder site.

We want to change the display the value added in the registration page in a particular customer field to another field.

Solution
1)Go to Commerce>Site builder>Themes.
2)Edit the theme you want to select.
3)In the general subtab search for the field Id of now using field.Then change the field id to the field where you want the value to appear.