I have a 2004 harley davidson 883 sportster and it keeps popping out of 2nd gear the guy i bought it from said that he replaced the gear and it just needs to be broken in?

I have a 2000 model sportster shifts through all gears fine when i get to 5th gear it makes a grinding noise

No, sorry to tell you that you don't need to break a gear in. There is something not quite right in the transmission if it's jumping out of gear. If I had to guess, he probably broke second gear speed shifting the bike, then replaced the gear and go the shimming messed up and it doesn't have the correct engagement with the shifter clutch dogs. To repair this, you'll have to pull the transmission out of the case. If your mechanically inclined, this isn't that difficult but you WILL need a factory service manual to check the transmission to find out what is wrong. Heck, he may have put the gear in backwards. Some of them will work that way but they'll jump out when power is applied or let off.

Good Luck
Steve