Will updating the subscription product on payment fields affect the previous subscription payment?

If I am doing a subscription based payment, can I go into the subscription and edit the amount and month, without messing with people that have already subscribed to a plan?

If you're referring to updating the subscription products on your form's payment field, it won't affect the previously received subscription payments. This is because it's not the form that manages the subscription charges but the payment processor to where the data from Jotform has been sent to.