"Layer shift" typically looks like your print started without trouble, and then a few mm up, the extruder jogged too far one way or the other before continuing the print. The problem can be tough to isolate, but it usually comes down to sloppy mechanical movement in the belts or pulleys. See the suggestions below and follow them in order when troubleshooting your prints.
1. Which axis is shifting?
Before taking the bot apart, take a look at your print to identify which axis is shifting. Side to side/left to right is X. Front to back is Y.
2. Tighten the belts.
After identifying the "problem axis", find the belt tensioner and tighten your belt. The GT2 belt on your Printrbot should be tight like a guitar string. You should hear a note when you pluck the belt. Tight belts = good prints :).
3. Check the pulleys.
If tightening the belts has no effect on the layer shifting, the problem probably lies with a pulley. If the motor shaft is slipping around inside of the pulley, the motor movement will not carry on to the belts, thus causing inaccurate motion on that axis.
Use a 1.5mm allen wrench to tighten the set screw down on to the flat side of the motor shaft. You will likely find that the set screw is very loose. TIP: Put a dab of threadlocker on your set screw when tightening the pulley down on to the motor shaft.
4. Replace the belts.
GT2 belts are made of rubber. Just like tires. ...and just like tires, they need to be replaced eventually. Over time, the teeth on your belts will wear down from all of the back-and-forth motion from hours of printing. If the teeth are worn down enough, it won't matter how tight your belts or pulleys are.