How can I make over an old medicine cabinet?

If it is the typical metal cabinet with hinged mirror, try adding a trim to the mirror. Home Depot and Lowe's sell all widths of trim in a plastic like compound that looks like wood but is lighter. They come all white so you do not even have to paint. Cut pieces to overlap the mirror about an inch and attach with Loctite power grip adhesive. Sticks immediately but gives you tons of time to move pieces around to keep straight. It should hold in place but if you feel better add a piece of painters tape to each piece to hold in place until the next day. Makes a huge difference - very inexpensive. One strip of this will do the whole mirror.

Most medicine cabinets have metal interiors, so I've jazzed my up with colorful sheets of craft paper just using magnets in a few areas. I can change the paper on a whim if I tire of the design. It's a happy surprise when you open the cabinet. A house-guest commented on what a fun idea this was. You can use most anything such as gift wrap paper, or print out a page of inspirational sayings, etc.

Qz

Paint and glue some interesting faux wood pieces on the mirror or the shelves. Wallpaper the interior also.

Here’s how we updated ours: https://www.frugalfamilytimes.com/2012/08/our-frugal-bathroom-reno-updating-old.html?m=1