Oh if it matters, its a Kee Trailmate edger model ET101.

According to user manual in my collection, for the SE101 and ET101 you simply loosen nut and remove washer to replace blade. It then says to reinstall washer and tighten nut.

I'm sure you figured that much out yourself.

If you remove the front belt guard and the belt itself, you may be able to grab the pulley on the hub with a pair of channellocks or vise grips. Do not grab the sheaves themselves or the teethmarks you leave will chew up belts.

Another trick is to get an old belt and a piece of pipe about 6" shorter. Thread the belt thru the pipe, one around around the pulley and put a bar thru the other end. When you twist up on the pipe, it will grab the pulley, allowing you to tighten the nut. Basically the belt and pulley are functioning like a home-made strap wrench.

I have a Kee Trailmate ET 101 edger that the torsion spring on the tilt trigger handle came off and I don't know how to re-install it. Any help will be appreciated.

Jwpl, I cannot help, as to the OP, help is easy..
Use an impact tool, or
Is the nut a "nylock" (plastic as a anti turn, better than lock washers) ??
It has to be tight, but NOT that tight, check the torque specs.
Another idea is the use of a "lock-tite" on the threads, but be careful here, be very careful. Some lock-tite is a forever thing.
Out in the field, the primary problem is overtightening, not under.