Overwrite a plugin

I need somebody who knows how to overwrite a plugin, we have a plugin called TI Template Invaders, they have a wishlist plugin and they made us a quoting system, the quote doesnt have the tax added an I need somebody who knows how to overwrite a plugin, we have a plugin called TI Template Invaders, they have a wishlist plugin and they made us a quoting system, the quote doesnt have the tax added and it doesnt have a email cc and customers cannot print as a PDF, instead of a link to the website wishlist, also the quantity b...