What is the best way to replace the footer.php via functions.php?

Hi guys,

I have a site which is uses Galleria by woocommerce. They suggest using a plugin to make customisation rather than creating a child theme (the Galleria theme is a child theme of storefont, I assume they are suggesting this so you don’t have a child of a child(?) ).

Currently I have edited the footer file to add an email sign up, change the credits, and make things pretty.

I know that I need that this will revert back to the footer for that theme with the next update. I have copied the edited footer.php into the customiation plug-in folder.

How do I tell wordpress to go and get that footer.php rather than the one for the theme?

OR

Whats the best way to replace the theme footer code with mine so it wont be replaced with future updates?

Thanks in advance for you help!