Depends on why you want to replace it. If it is broken beyond repair I bought this as it allows you to run 7-9 speed Shimano cassette/freewheel and can also handle up to a 45 tooth granny gear not to mention some other benefits over the Tourney derailleur that came with the bike. Great deal at $16 https://www.walmart.com/ip/488889285
There are 8 speed freewheels available so it would set you up for an 8 speed upgrade in the future if you ever want to replace the shifter & freewheel.
By far the best change I made to our stock XP2.0 bikes was putting in a 34-11 freewheel, which the stock derailleur handles fine. Major improvement at both ends of the range.
