How easily can I remove/replace the nozzle of the prusa i3?
Assuming you have a typical setup:  nozzle threaded into heater block, use a socket wrench on the nozzle and any wrench or vise-grip on the block, and unscrew the nozzle.  Now, if your system is fully clogged, you may want to heat the head to melt all the residual gunk (which otherwise will strongly resist your unscrewing force).  Obviously take care not to burn yourself if removing the nozzle while hot.  

Alternatively,  don't bother removing the nozzle.  Raise the extruder as high (Z-axis) as you can, heat, and use a properly-sized stiff wire to ream out the nozzle.  You can use the back end of a microdrill bit.  Hot or cold, you can use the drill itself to ream, but be very cautious as you may widen the nozzle itself doing this.  

I should add that you may well have gunk in the feeder tube, and that's considerably wider diameter. Remove the nozzle and gently drill out the feeder with a matching drill.