valves and springs?

how hard and could i do it. Can i replace the springs that are under the rockers with out any problems if i find out how to hold the valve in place.. its come down to replaceing everything testing and everything... so im thinkin one of my valves doesnt have enought umph to close all the way.. so im thinking if i get new springs then maybe i could have a smooth idle? i mean the car is smooth as hell above 1500 rpm...