How can I best update my sun room on a budget?

Wow I just did my sun room... I used SW agreeable gray on my wood walls. Oxford white on the ceiling and trim.. Boy I did have a lot of trim.. For the floors I used a flooring I got at Home Depot. Comes in strips and self stick. Looks like hard wood floors...Wish I could remember the name of it. But it looks like a all new rooms. Good luck!

Paint, glorious inexpensive paint my dear. Sounds like a great room to work with. You didn't say if the ceiling was raw wood or drywall but if it's raw wood and you like it, leave it, or you could whitewash it just lightly so it looks beachy. I would paint the walls something light and pastel like a palest blue maybe (pool) or some color you love, it's your room after all. :). You didn't mention any window covering so I'm going to "assume" it's not a priority or you have blinds or nothing. If there is someway to to attach them at the corners of the windows, add white sheer billowy clusters to just hang there and soften the corners, they could even puddle on the floor. You could even use sheets for no sewing or big expense. If you wanted to hide where the windows meet the wall a complete wall to wall valance would look good and you can make them yourself with cardboard or styrofoam and cover with material to complement the curtains or walls.

Have fun, I wish I was there to help, I love this kind of thing. :)

Here are some beautiful ideas: https://www.bhg.com/home-improvement/porch/sunroom-decorating-and-design-ideas/

Hello, you could whitewash or stain the wood walls, this will lighten things up a lot, you can lay new floor over the old , just fill in any divots with filler first. You could even paint the flooring once you level it