Your primary account can be changed anytime you like. You can change it by doing any of the following:
1. Access your profile by clicking on your avatar in the top left of the home screen→ Once you are in profile, click on banks and cards menu → Select any account and an options panel will appear allowing you to enable Primary account.
2. When conducting transactions, a 'paying with' section on the transaction review screen will allow you to change your primary account to any of your added accounts on the fly.