I was trying to download a sims 4 save file and it kept on showing up as a minecraft file. How do I change it?

It's likely ok.

Windows will register extensions (.txt, .exe, etc.) to a certain description and often to a program so if you double click it, it will open a specific program. That is why when you double click on a .txt file, Notepad opens.

Minecraft must have registered whatever extension is used for its save files.

Sims 4 must be using the same save file format, so it's showing up as a Minecraft file.

But if you place it in the correct directory and launch Sims 4 to have it load the save file from there, it should work just fine. It just means that Sims 4 isn't registering the save files as their own to the OS. No harm in that.