changing button text using filter

Thanks for great plugin. it works perfectly as i expected.
Here i want to change text of button “add to cart” to purchase and and “make purchase through paypal ” to make payment .
currently i got that editing plugins file which is not good practice.
i got admin’s mail suggesting to use filter to change text.. i am not good at using hooks and filter .
so please can you give a idea how to use filter to achieve that

https://www.ads-software.com/plugins/my-tickets/