Assuming you are referring to a car's turn signal, there are a few things that could be causing the issue. The most common cause is a burnt out bulb. If this is the case, simply replacing the bulb should fix the problem.
If the bulbs are not burnt out, then there could be an issue with the wiring or connection. This will require some troubleshooting to figure out where the problem lies. First, check all of the connections and make sure they are tight and free of corrosion. If everything looks good there, then you may need to trace the wires to see if there is a break or loose connection somewhere along the way.
Fixing a fast blinking LED turn signal can be a simple fix or a more complicated one depending on what is causing the issue. By troubleshooting and ruling out each potential problem, you should be able to narrow down and fix the real issue.