How do you change a mower blade on a troy-built model 34345?

Remove spark plug wire, better yet remove the spark plug completely, hold the blade and use a wrench to remove the bolt.

MM:
I don't like the wrench idea without a little more info. More specifically, I'd recommend a 6-pt socket at a minimum. 12 pt wrenches have a tendency to round off the heads, so use a 6 pt wrench.

Preventing the blade from spinning can sometimes be done my holding the blade. Occasionally you'll have to wedge a piece of wood between the blade and the housing.

Otherwise, mm suggestions are spot on.