The early Universal was more or less an exact copy of the GI M1 Carbine, and the very early rifles actually used new or refinished USGI parts.
A trigger job is done by very carefully stoning the sear and hammer interfaces, and possibly installing a lighter hammer spring.
The trigger spring is already about as light as you'd want to go.
Take notice that stoning the Carbine trigger is something not that easy to do properly, and its common to have failures to cock, TOO light a trigger, or slam fires when its done wrong.
The best source of info on getting a better trigger is to buy a copy of Jerry Kuhnhausen's book "The US Gas Operated Carbines: A Shop Manual".
This is a real gunsmiths manual on the Carbines, not just the usual reprint of GI manuals.
This covers EVERYTHING on the Carbine.
Since the Universal was a GI copy, the info is applicable.
Money well spent if you own a Carbine:
http://www.brownells.com/.aspx/pid=..._CALIBER_GAS_OPERATED_CARBINES__A_SHOP_MANUAL
