Having a bigger dick is overrated. Great for fantasy, but very impractical.

I’d upgrade my lungs to be better filters and oxygenate my blood. My immune system to fight diseases my efficiently (turn it into a scalpel rather than a cluster-nuke). My brain to retain it’s learning capabilities well I to my later years. Completely overhaul my metabolism so that I won’t have to do heavy exercise to get good muscles (really, all that exercise just exists to make one chemical reaction happen).

After that, then, only then, might I consider doing something with my dick. Though perhaps more with my balls, so I can just switch off the reproductive capabilities to ensure safe sex.