Talk to them and admit the Screw up.
Either let them build down to a bit above the theoretically current XP (like if they have about 6000 now, they should have 1500, so let them have 2000), or shortchange the XP for coming encounters (give out fewer than you should, maybe up the CR and don't increase the XP gained) until they are where they should be.
Everyone's a newbie when they start. Admitting screw ups is what grown ups do.
It's a little too late to take it back, and it would be very annoying to un-level characters. Just let them know you made a mistake and withhold XP until they would have actually acquired that amount of XP. You might also include some additional creatures to make combat a bit more challenging for the party's current (high than expected) level while no providing extra XP for it.
I also agree with Franz.
Just explain the new (proper) XP devision and reduce upcoming the XP of creatures until their lvl and the CR match.
Probably they are nearly as new to it as you are so they might not mind just give it some fluent tweaks on CR, like give a light boost to AC or att to creatures in the coming fights not to make it too easy.
Remember you are the DM, your word is law.
Best way to really reduce XP is in any boss encounters.
Ours did the oposite and put a bit too high of a CR at first, but just gave us a quick random XP bonuses to catch up giving the encounters light debuffs. And giving us a bit more advanced loot to start. (he did incorporate the tweaks in the story giving us devine XP bonuses and the gear was found "helping" *cough*looting*cough* dwarfs defend their city from an invasion.
Worst case, you could kill them all and revive them with a negative level. Lets say the grand master evil man wanted to make sure they would remember him for he still needs them in his evil plan for now.
EDIT: Also blame your henchmen.
I would recommend admitting the mistake and then going to slow advancement until they have advanced to the point that the monsters are the correct level. In other words, let them stomp for a bit and keep the monster challenges as is, but they don't get full XP.
You are new and so presumably are they. Mistakes happen, especially at levels 1-2 where suddenly a PC can go from full health to dead. I've always found level 3-4 to be much more comfortable since you are tough enough that one hit generally doesn't take you out.
By allowing them to stay at level 3 they can more easily defeat the challenges, but it also means that as the challenges get larger they have some room to grow their tactics. Hopefully whatever you are running has a variety of encounters -- swarms, ranged combat, grapples or other CMB/CMD combat.
They are tough enough to survive some major tactical screwups. Hopefully this gives them a chance to learn from them without getting killed.
I would recommend admitting the mistake and then going to slow advancement until they have advanced to the point that the monsters are the correct level. In other words, let them stomp for a bit and keep the monster challenges as is, but they don't get full XP.
You are new and so presumably are they. Mistakes happen, especially at levels 1-2 where suddenly a PC can go from full health to dead. I've always found level 3-4 to be much more comfortable since you are tough enough that one hit generally doesn't take you out.
By allowing them to stay at level 3 they can more easily defeat the challenges, but it also means that as the challenges get larger they have some room to grow their tactics. Hopefully whatever you are running has a variety of encounters -- swarms, ranged combat, grapples or other CMB/CMD combat.
They are tough enough to survive some major tactical screwups. Hopefully this gives them a chance to learn from them without getting killed.
Just as BertI said, use this starting time as a tutorial for both you and them.
Also good if they get a bit cocky, all the more delicious to see their dispair later on when things will get tough on them.