White Poinsettia The Fabric Of An Old Cross Stitch Project Has Turned Yellow With Age. Any Idea On Getting Rid Of The Color? ?

The fabric of an old cross stitch project has turned yellow with age. Any idea on getting rid of the color? ? - white poinsettia

I sewed a wreath poinsettia fabric white cross. I want to be careful not to ruin the fabric or the design of the cross-stitched. Please give details of materials used and how. Thank you in advance.