I know this is a simple thing to do but I keep forgetting, so here are the instructions again:
1. Find a file that is the file type that you want to change the default application for (eg. jpg, mp4, pdf, etc.)
2. Command-i to see the info about this file
3. In the section where it says Open With, choose the application you want to use as the default
4. Directly underneath where you just changed the application you’ll see some text that says “Use this application to open all documents like this one” and a button that says Change All – click this button
Voila! Now all of those files of the same file type will automatically use the application you’ve just chosen as their default.
Ta da! Very simple, like I said.