You have 2 choices
1) manually create a new field in the fields array portion of the code.
If the field is similar to one of the existing fields you can just to a copy and paste then change the things that need to be different. Note that when you do this the “key” value must be unique, it should start with “field_” and can be up to 63 characters long.
You can also create what you want to add using the ACF interface, do an export to PHP and copy only the field array that you want to add to the fields array.
2) You can completely recreate the field group using ACF, using the same field names listed in the code. Then delete the existing field group from the code.
3) You could also to a combo, create a new field group and export the code to replace the existing code with.
I’d probably to the first one. an way you look at it, if this is a theme that is supposed to allow updating, unless you can create a child theme, you won’t be able to update it.
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