Is the field group location rules set to show on attachments or somewhere else?
Before you can update fields on attachment you need to create a field group that adds them there.
Then, where is the update happening? When you edit the image or when you edit some other post? I’m assuming that it’s when editing another post. In this case you would use an acf/save_post action and in this action you would update the fields attached to the image after getting the attachment ID you want to update.
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