Marketpress change 'Taxes' text to read ' GST '

hey there

I need to change the wording 'Taxes' anywhere they show up on the front end to instead say ' GST '

This is as per our Aussie laws

It needs to be a permanent change, and not be overwritten when the plugin upgrades as I have the plugin installed on a number of sites and don't want to have to tweak each one whenever there's an upgrade.

cheers

Stef