Chlorine bleach is great for cleaning and disinfecting but it can cause yellowing if used on white synthetic fibers. The bleach weakens the fibers and returns the synthetic polymers back to their original color - yellow.
Some stains just can't be removed but, your best bet for saving a yellowed white garment or linen is to use a color remover. Rit Color Remover is a wonderful product. Just follow the package directions.
If you love the clothing and can't get the yellow out, consider dyeing the fabric another color. Today's dyes are more color-fast than those of years past. You should be able to enjoy the garment for many more wearings.
Specific questions? Just ask here.