There is a small screw in a recess at the lower end of the cover.
You will need to remove this screw before you do anything else.
Once the screw is removed, the lower end of the cover is lifted up first because the end near the carb is like a hook that is comes off only with the lower end lifted up.
The throttle trigger is fitted on a small drift pin which has to be knocked out.
Once the new throttle lever is fitted, and if you dont care too much trying to catch flying objects, move the control switch to choke with the throttle lever depressed.
This should hold both the the spring loaded triggers depressed and in place while you try to get that cover back in place as daintily as stepping over a dozing lion.
Watch out for flying objects at this time.
Once you get the cover back in place you can let go that breathe you've been holding so long.
You now need to get that screw back in, that is all .