How do I change the file type?

@BatmanFanGirl  This isn't actually a problem from the game's perspective: Sims 4 will read the .package files the same way as it always has regardless of what file association Windows has applied.  But if this bothers you on an aesthetic level, the only "fix" is to choose a different file association.  For example, you could download Sims4Studio and set .package files to be opened with that.