I am wondering what would happen if I import ACF fields into a site, where the field group is already existing?
I have a website and a dev-copy of the same site. When I do some changes on the dev-version I want to export the fields and import them on live. Will it correctly update existing fields or should I manually create the fields instead?
It depends on how the sites were built and how you do the export/import.
Where the field groups exported and imported do that they are identical? If you created fields manually in both then you’ll have trouble importing one into the other. If they are identical then you could use the standard WP export/import, but you may loose some things, like media.
The best way to export and import and ensure everything is the same is to go into phpMyAdmin, export the entire database and import the entire database into the other site.