If you change the price of an existing subscription, this will affect new users and existing subscribers as follows:
• New users will see the new price immediately and will be able to subscribe on Google Play.
• Existing subscribers will be notified by email and by a notification on Google Play 30 days before the price change takes effect. The date of notification will depend on the length and expiry date of a subscription. If subscribers wish to keep their subscription, they must agree to the price change on Google Play.
• Price changes do not take effect immediately for existing subscribers. After you change the price of a subscription, you have at least 7 days to notify your subscribers of the price change before notifying them on Google Play. Subscribers can agree to price changes before they are notified on Google Play.
• Important: If subscribers do not agree to the price change, their subscription will be cancelled on the renewal date when the price change would take effect.
You can find the detailed instructions here: support.google.com/googleplay/android-developer/answer/6334373?hl=en