Help Please - how do I change the text of a popup error message

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I am using Recharge, and when customers try and use the BUY NOW button for a subscription product they get the message below popup - this will stop orders, so I am looking to change the text to " TO SUBSCRIBE, PLEASE ADD TO CART AND CHECKOUT" 

I am happy to modify code, just don't know where to start 

Thanks for any help 

my website is 


Screenshot at Jun 16 10-10-20.png