First you must remove the old toilet paper roll
Pull back on the bar in the roll, like above
Once you've removed the bar, remove the old roll and discard it, be in the trash or Recycling, the floor is not a good idea!
Place fresh roll onto bar, being sure that the toilet paper will pull in the way you desire, over or under, we use over.
Place bar with toilet paper back onto holder
And there you go! If you're really lazy you can do this while sitting on the toilet!