The first thing to try, prior to any surgery, though is to adjust the variable resister (also known as a potentiometer, or "pot") on the wire feed circuit board. It should look like one of these;
Pull the wire back out of the torch so you don't waste any, then very carefully (because there is mains voltage on that board) use a plastic handled screwdriver to adjust that pot while pulling the trigger. Try in both directions until you have managed to slow the speed down to as low as it will go.
Given the potential danger, it may make sense for you to get someone else to be pulling the trigger while you concentrate on adjusting the pot.
Finally, if it's still too fast, a number of people (including me) have replaced that pot with a lower-value one.
Found a better picture. It's the wee black chappy on the right. Do not remove the PCB to adjust it - adjust it in situ as that's safer than having the PCB flopping around.