How do I replace the ceiling in a 20+ year old mobile home?

If it's your style, you could use "tin" ceiling tiles. They are actually made of pressed plastic and come in a variety of styles and colors. I also believe they can be painted-maybe try an unusual, unexpected paint color. They are found at a big box home improvement store.

You can also put up drywall over the panels.

you MIGHT be able to remove panels & bleach them and or paint them after using Kilz on stains

Depending on what's ugly about it, you might try painting it. Most materials can be successfully painted if the right product is used.

If it's a dropped ceiling with panels that come out, it is best to remove the panels before painting. While the panels are out, clean the tracks well. If the tracks don't clean up well, use a foam roller to paint them with a paint meant for metal. The panels themselves are likely some kind of fiber material. If they are the soft kind that are basically a plastic film coating on a thin layer of fiber insulation, those will be harder to paint and you would probably have to spray paint them after cleaning well. If the more solid kind, those can be painted with a roller.

If it is a dropped ceiling, maybe try doing something really creative instead of what is expected. Copper (or other metallic) painted tracks with panels in a color to match your decor would be unusual and fun. Or painting different panels different colors for a patchwork effect would give a different look. You could even paint designs on the individual panels, though you might not want to do all of them - perhaps a central section of four or six to make a picture. You could even decoupage a poster you cut into pieces or some wall paper or something onto them.

If all those ideas don't appeal, you could cover the panels in fabric and put them back up or even replace the panels with sections of paneling or beadboard or lauan cut to fit, if the track is strong enough. You would want to make sure the replacements aren't much heavier than the original panels unless you increase the strength of whatever is holding up the tracks (usually some sort of wire). I am not sure how you would do so, but it could even be fun to replace all or some of the panels with pegboard with lighting above it. It would look a bit like stars at night if you covered a bunch of the pegboard holes with masking tape or something so the open holes mimic constellations.

If the ceiling is not a dropped ceiling with tracks and panels, I am not sure what kind of panels it would be, but if they come down, they can be cleaned, painted, or replaced. Replacing such things can be super expensive, though, so far better to clean and/or paint and/or cover with something if possible.