When updating from an older version of a livery pack to a newer one personally I'd delete the old one first to make sure its a clean install. You could just drag and drop and choose the "replace all" but I tend to delete the older version to make sure it doesn't bug out or anything.
If its two different livery packs by two different people you can have them both installed at the same time, all duplicate liveries will be shown when selecting the livery in the editor. You can just delete the duplicate ones you dont want if they're worse.
