We've no official functionality for mass modifying subscription dates like that. Assuming you're using non-recurring payments the easiest way to do this is to increment their subscription expiration date by 1 year directly in the database. The below query can accomplish doing this for you.
Code:
UPDATE `TABLE_PREFIX_cbsubs_subscriptions` SET `expiry_date` = DATE_ADD( `expiry_date`, INTERVAL 1 YEAR ) WHERE `plan_id` = PLAN_ID
Replace TABLE_PREFIX with your actual database prefix. Replace PLAN_ID with the plan id you want this to be applied to. So for example I might have the following.
Code:
UPDATE `jos_cbsubs_subscriptions` SET `expiry_date` = DATE_ADD( `expiry_date`, INTERVAL 1 YEAR ) WHERE `plan_id` = 1
If your subscriptions are recurring subscriptions you've no choice but to first force all the recurring payments to be cancelled. If you're using Stripe you can actually pause payment collections for subscriptions, but you won't have this functionality for other gateways and this must be done from your Stripe dashboard. If you are using Stripe and paused the subscriptions you can use the above query to push their expiration date.