How complicated is it to lighten this trigger? It breaks at about 5.5 lb. would like it at about 3.5.Any comments would be appreciated. Thanks

The design of the Savage 99 trigger does not allow much. The best you can do is to smooth the surfaces that engage, and most likely, time has already taken care of that.

The trigger return spring is very heavy in the 99
It's a simple short flat spring held in place by a small flat headed screw.

Take the butt stock off and you can easily see and get at the spring and it's screw.

One easy 'trick' to lighten the 99 trigger pull is to lighten that spring up.
To do that w/o trying to thin the actual spring (grinding, ect),,is to simply remove the spring and place one very small flat washer under the spring and reattach with the screw going through the washer and the spring.

That elevates the spring slightly and takes some of it's tension off of the trigger.
The felt trigger pull is lessened by usually a #+.

Some people have placed 2 washers under the spring and of course lightened the pull even more.
Depending on how thick the washers are, you may run out of threaded shank on the retaining screw to hold everything in place using 2 washers.

There's no touching the sear surfaces with honing stones, none of that. Leave that all alone.
If the engagement is reliable as it is, the trigger return spring is not what holds the sear in place, it is the geometry of the angles involved.

cutting thru the blather, when you take the stock off you can see the contact between the trigger and sear and it usually a bunch..pull the trigger and you will see how much drag there is..cut about half off with a file and polish. cut at a very light angle..then harden if you so wish (this takes removal of the trigger) I just polish then try until its good, don't go to far, but if you do just buy a new trigger and start over...A 5 year old could figure a 99 trigger out...its not rocket science..and at 3 to 3.5 seems to be where I usually end up..You don't even have to disasemble the workings btw..All you cut is the sear thats part of the trigger that points UP and slides up and down..Ive done maybe maybe a hundred over the years..Be sure and bounce it off the floor if it has a recoil pad, if not the thump it on the bench..if it triggers you cut too much, but 3.5 will always hold...Its not a bolt gun, and the triggers are seldom that good, but they are fine for hunting..The engagement from the factory is always near 1/8 or better inches, and thats a bunch..and has to be cut in half as a rule..light polishing of the surface doesn't hurt a thing as it in many cases has burrs.