Depends if it's a star nut or expansion plug.

Starnuts can sometimes be broken, or turned sideways and eased back up. generally it's easier to push them deeper and out of the way and use a new starnut.

Expansion plugs should be disassembled and the top part lifted out, then the expanding sleeve or shoes can often be eased out by sticking a finger in and working them up. Many of the types that only have a single split are difficult to lift out this way, so I use an old screwdriver that I bent into an "L" or open "J" shape years ago to get behind them and pull them up. Lastly turn the fork over and shake the expanding cone out.

The one thing you usually can't do with expansion plugs is pull or push them out assembled, since any pressure applied to the expanding cone expands the sleeve and locks them in.