Nick Bartlett“Every man I meet is my superior in some way, and in that I learn from him.” - Emerson

In short, you don’t.  I searched high and low for this simple feature and apparently it does not exist.  I contacted Paypal and asked where the functionality was.  Their response:

“Once existing subscriptions are in place they can not be changed. You
must close the existing transaction then create a new one for the proper
amount.”