The first step is to climb a ladder and clean the leaves out of the gutters. The decomposed leaves make great compost. If leaves and debris are clogging it, water won’t drain properly and so along with mildew and mud you’ll end up with sagging gutters.
Another thing you want to look at is where are the sources of any leaks, including holes in the gutters and cracked silicone. Use an old scraper to remove the old silicone and dry the area thoroughly. Then apply new silicone to keep water from getting behind the gutters and rotting the boards.
The best way to clean the gutters is with a pressure washer, which you can rent or even purchase. Follow the instructions that come with the washer to the letter; usually the washer can be used with garden-hose water and some degreaser.
If the gutters are rusting, get all the rust off, sand them down, paint them with Panache Anti-corrosive metal primer. Seal the inside with a Flash harry fiberized drip stop. Then paint the outside after primer with either a Panache Roof acrylic or Water based High gloss or Polyglo.
Happy painting and keep those gutters clean!