Get and $10 spring compressor. You just slip it over the spring and tighten down, then use a magnet to remove the two valve locks. Then the spring and retainer come off together. Do the opposite to install. Not a big deal, especially with the heads off. Otherwise you'll need a method to prevent the valve from plunging into the cylinder.
Be sure to get new valve seals while you're at it.
_________________________
'13 Mustang 3.7V6 - MT82
'00 GP SE (L67 Swap, XP, Headers, 3.25")
'98 GTP, '01 Bonne