I would like to change the error messages

Specifically I would like to change this message:

"The cart items combined weight is too big for all the possible shipping methods"



Where do I find this in the Hikashop code? Thank you!

You don't need to change the code for that.
You can change any text in hikashop by overriding it via the translation files edition interface:
www.hikashop.com/download/languages.html#modify

Thank you!