I want to get direct access to the top contact of an LED. How do I remove the layer of resin on top of the LED?
This is called Dedoming the LED and is usually done with careful use of razor blade and/or abrasives. Be sure not to remove the phosphorus and consider just starting with an un-domed LED.
The normal reason to do this would be to increase the throwing capability of a flashlight. Unless this is exactly what you're trying to achieve, you may wish to keep the dome or just order a different LED as it might decrease your beam quality (uneven coloration, beam shape problems.)
Assuming you're trying to remove the plastic and make contact with the die, I would try the standard methods for decapping an IC, after securing the leads so that the bonding wire won't get pulled off. And mill the top so there isn't so much to remove, trying to leave a depression for the fuming acid that will be required (do a google search for the chemical process involved, I don't think that belongs here- a few folks including YouTubers have done this and described their successes and failures in some detail, including safety precautions, which are vitally important with such dangerous chemicals).
You might also consider a metal can packaged LED (hermetic) which is much easier to open up.
Try methyl ethyl ketone aka. Butanone. I have removed plastic casings with it. Unfortunately I have not tried it to LEDs.
See this Wikipedia article:
Using solvents by simply sinking an item into the liquid isnt't generally a good idea if you want leave a well controlled part of the plastics in place. Mechanical crafting (incredibly difficult) should be used and the last few 1/100th millimeters can be dissolved. I found soon that this job is better to be left for non-bunglers.
I have ground down the sides of two identical LEDs to make a (somewhat oblong) single bi-directional IR LED. A small crafts belt sander works well in this situation. If a fine finish is desired, sanding with automotive fine-grit finishing sandpaper (grit in the 1000s) and further polishing can be done.