I want to change my subscription text "Subscription interval: 30 days" to "Deliver Every Month".

You can easily change this in Seal Subscriptions > Subscription rules. In there, just click on the subscription rule and adjust these values in there. 

The “Deliver every” text can be set at the top of the subscription rule configuration and the delivery interval (e.g. Month) can be set on each selling plan which is configured in the subscription rule.