Restoring a faded thermal paper is quite simple. Here are 3 ways to do so.
1 – Scan and digitally restore it (as everyone else mentioned)
Assuming that the paper’s surface is still white (not yellowish, brownish or blackish), scan the receipt just as you would a colored photo. Open the image in a photo editing software, like Adobe Photoshop, and make a negative of it by adjusting certain image settings. You should find these options in the tools provided in the software.
2 – Apply heat to it.
If you’re no Photoshop genius, you can try restoring it the old-fashioned way: by gently applying heat to the thermal paper.
Set your hair dryer on medium and then blow hot air directly towards the receipt until the text becomes visible. You can also heat the receipt using a light bulb if you don’t own a hairdryer.
Regardless of the heat source, never try to apply heat on the front as this will cause the whole thing to turn black. Keep the source of heat a few inches away from the BACK of the receipt and make sure to pay close attention to whether the heat is reaching the faded paper or not.
Also, you may want to wash your hands or wear protective gloves before handling the receipts. As mentioned, water and oil (the natural oil produced by the skin included) can contribute to fading. With oily or sweaty hands, you might get distorted text and images, rendering your efforts to restore the faded receipts useless.
3 - Coat it with a heat-insulating material, or replace the thermally-sensitive ink with thermally-insensitive ink
Also I read from those who experimented that by applying highlighter as we do to highlight something on a paper, it will apply a thin coating on the receipt and it never fade.