Click Memberships > Subscriptions and Packs > click the subscription in question
Scroll to the bottom, below 'What's included' > to the right of the customer list click the dropdown menu with three dots > click Bulk edit active members
Choose the new renewal price everyone will pay. You'll have choices between how to edit the price:
Specific: Change the price to whatever the new one will be. Just list what the new price is.
Absolute: Increase the price by a specific amount. Ex. put $7 to increase to current subscription price from $50 to $57.
Relative: Increase the price by a percentage amount.
With both absolute and relative, you can use negative numbers to decrease the amount of the subscription.
