If you are an iOS user:
1. go into your setting on your phone
2. Click your name at the top to get to your apple ID and iTunes
3. click iTunes & App Store
4. click your apple ID at the top then click "view ID"
5. Click Payment Information to check to see if the correct credit card is linked to your iTunes!
If you are an Android user:
1. Go to Google Play
2. From the left sidebar menu, select "Account"
3. Click on payment methods to add, change, or delete a credit card
If you are a web user:
1. Log into your GZ profile at www.glucosezone.com
2. Click on the GlucoseZone Menu button
3. Select "my account"
4. Update your Credit card information