|To take the yellow out of decals:|
You'll need a window that has the most exposure to the sun.
Take a piece of cardboard, then place the decal sheet on it, and slightly tape the decal sheet edges, then tape the cardboard on the inside of the window glass. This will keep the decal sheet from possibly curling. Make sure the window doesn't get moisture on it as it does sometimes during the winter months, otherwise your decals will be ruined! In that case, put the decal inside a tightly-closed transparent plastic bag (although that does limit the sun's effectiveness somewhat)
It may take from a couple of weeks, to a couple of months to fully clear them, depending on the sun's power in your area. Then again, they may not clear at all. The best way to do it is to try it. The older the decal is the harder it may be to clear.
Note that this removes the yellow from the clear film, not the yellow from white ink; it may help some, but most likely if the white areas on your decal has yellowed, that's it pretty-much.
However, when done, there is no quarantee that they will not turn yellow again with the passage of time (keeping them in the plastic bag wrapped in paper helps slow this down, see below) but the procedure can be repeated. You'll need to prevent oxidation, which created the yellowing in the first place, so DO THIS RIGHT BEFORE BUILDING THE MODEL, then put them on, and when dry, seal them right away, with Future floor polish or varnish or whatever gloss coating you use.
If you don't want to build this model, but want to keep it for your collection, you can perform this procedure to remove the yellow, then return them to the storage bag, wrapped in the printer white paper, inside the zippered plastic bag. This should keep them fresh for a long time. As a matter of fact, I would suggest you do that with all the decals & instructions you have of opened kits. The plastic bag keeps out dryness and/or moisture, while the paper wrap provides enough miniscule moisture to the decals' & instructions' paper pulp so they will not dry out.
To straighten decal sheets, wrap them with a couple of printer paper pages and place them between the pages of a big heavy book and leave for a week or so. They will absorb enough moisture from the printer paper to straighten up (mostly) If the sheet is severely curled, boil some water (the tea kettle is perfect for it) and hold the sheet above the steam (not too close!), and they should start slowly un-curling. Then, take some plastic wrap or a ziploc bag, place it over and under the decals sheet, then place it between the pages of a heavy book. You don't have to keep them more than a day or two in this case, as they have enough moisture, and due to being in plastic they may develop mold. In that case, use rubbing alcohol with a cotton ball to clean the mold. Repeat the steaming until they are straight. When straight, let dry between the book, then use the printer paper to wrap them and place in a sealed plastic bag to keep fresh.