Here's a guide to do this.
Step 1: Download the firmware update for your flashcart.
Step 2: Extract and put the firmware update on your microSD card.
Step 3: Put the flashcart into DSi, DS Lite, or DS Phat, or a 3DS that wasn't updated since the flashcart was blocked.
Step 4: Run the firmware update. Make sure you have a charger plugged in, or your flashcart could be done for.
Step 5: Wait until its done updating. When its done, turn off the DS, and turn it back on. If your DS/DSi says no cartridge(No Nintendo logo on startup), your flashcart is
busted. I hope this helps!