To replace broken valve springs or defective Valve Stem Oil Seals without removing the 
Cylinder Heads, keep these steps in mind, First, take off the valve cover(s), and both should be removed if all Valve Stem Oil Seals are to be replaced. For all seals replacement, remove the spark plug from the cylinder having problem or remove all of them. Turn the crankshaft until the affected piston is brought to top dead center on compression stroke. Start by cylinder number 1 in case of replacing all seals and follow through with cylinders following the firing order. An adapter is threaded into the spark plug hole and an air hose connected to a compressed air source, holding closed valves. Insert shop rags into cylinder head holes to keep debris from dropping into the engine. With a valve spring compressor, compress spring/damper assembly and remove keepers. Lift out spring retainer/oil shield/valve spring assembly along with intake valve's O-ring seal on valve stem and umbrella type guide seal. Wrap a rubber band or tape around the valve stem to avoid it falling into combustion chamber and release pressure of air. Check that there are no damages on valve stem and make sure it moves freely in guide. Lubricate valve stem and fit new intake valve umbrella-type guide seal for intake valves insertion point where it's stuck at top position around a piece of dowel pin that fits inside of it via available range at bottom of spring retainer channel. Install d/spring/damper assembly & shield compressing it enough for you then install n/O-ring seal over v/stem; Position either v/spring retainer or rotator; place "keepers" using dab of grease for retention till pressure is taken off s/tool; Disconnect air hose; put 
Rocker Arm(s) & 
Pushrod(s) then that may be followed by reinstallation of valv/e covers; Reinstall spark plug(s) & wires, start engine and run engine also check for any oil leakage or any unusual sounds around the valve cover area. Be advised that these procedures require specialized tooling and should be performed with caution.