How do you clear the transmission limp mode warning light from 1992 Xj6 Jaguar?

There are several ways, one you could try turning the car on and off, many times this will cure it. If it doesn't, drive the car around for a bit and so long as the problem that caused it to go into limp mode disappears then when you turn the car off and on once again, limp mode will also disappear. The last resort is a hard reset. Disconnect the negative battery terminal and touch it to the positive terminal and this will reset and clear the code.