What should I do to improve my old weathered deck?

You need to clean it well and then put a coat of wood stain on it - what ever colour you want. You can buy cleaning products specifically for decking, but warm water, dish soap and a deck scrub brush work well - do rinse well and allow to dry before staining. Good luck!

Do not use a solid paint or stain as it will peel. The pros on "This old House" recommend using a semi-transparent stain. Pressure hose and let dry for a couple of days so that it is thoroughly dry and then paint.

https://www.wikihow.com/Seal-and-Stain-Pressure-Treated-Wood-Decking

I pressure wash my deck once a year, Kathy. I love the wood so I only use Thompson's water sealer on it. Pressure washing it makes it look new again! Good luck!

I did this to my 17 Year old cedar fence. Colored stain. It came out looking great!

I would clean and seal to get a good protective coat. Then flip all the boards over to the other side and re stain. Brand new look.

water blast it or sand it down to bear wood and then seal it with an oil product available from your local hardware store.

I scrubbed mine with a wire brush ( a lot of work, but it looked like new) and then coated it with Thompson's water seal to protect. As it ages again, I will be painting a carpet design over a section to give it a new life once again. The sun will mellow any colours you choose, so depending on your desired end result, seal appropriately (gloss, matte, several coats etc)

I sanded mine with 80,120, 180 sand paper on my electric sander and then stained.semitransparent looks great...