1. To make any account the default primary account be sure it is the first account listed under SETTINGS, ACCOUNTS, GOOGLE.
2. If it is not the first account listed you can delete and re-add to make the next account on the list primary.
3. If the account you wish to be primary is not the default account you can also change within the GOOGLE applications.
For example, if you select GOOGLE PLAY and then the MENU bar (3 vertical dots) then select ACCOUNTS you will be provide a list of your current accounts to select from. This selection will remain the default primary account for THAT application.