When you change your email or password at one (or more) of the stores, you should update them in BookTrakr in the Stores tab of the Settings screen (http://www.booktrakr.com/stores). Just click on the Edit button next to the store you want to change and enter the new email and password, then click on the Save button.
When you change your Google Play email or password, you don't need to do anything in BookTrakr since we don't use your credentials there.
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