I edited the current instance, but didn't manage to edit all future dates. The next instance is too far ahead to see on the upcoming bills.
Actually, I figured it out by going to the calendar. Unnecessarily difficult!!!
You don't need to use the calendar. Go to Bills & Income > Projected Balances. Click the leftmost dropdown and select Next Instance. Choose the account in the 2nd dropdown. You should then see the next instance of all scheduled transactions in that account.
@Marian Hill
While they work, I find both of those ways a bit convoluted.
Just find the next instance of an unposted reminder wherever you like**. (The light gray transaction with a clock.) Double click it.
You will see options drop down for the transx: just know the behavior of each:
"Mark as Paid" will take you to THAT instance about to be posted. Edit it to your liking, and hit return to post. It only affects that instance.
"Edit Schedule" will take you to the scheduler where you can change the amount, frequency, etc. That is what you want if you want to use if you want to update all future instances.
"Skip"=this instance.
"Delete"=the series so the reminder gets removed.
**RickO provided smart advice to use the Projected Balances view as this will allow you to see all next instances without meddling with an account's settings.