Changing 'Shopping Cart' to 'Rental Cart'

Hello, I’ve been trying to change some words on the website. For example, I need to change ‘Shopping cart’ to ‘Rental Cart’ because my website is more on rental instead of shopping.

Could you please guide me as to which file I should take a look at? I’ve tried playing at both template_functions (bad choice since I’ll be upgrading every now and then) and mp-default.po although none of them provide me with the solution.