Epic support is your friend!
I had a lot of trouble getting two of these registered to the correct epic games account - I guess my kids have a couple of accounts over different devices. Epic games support was amazing at resolving it - just had to email a screenshot of the Fortnite account number (you can find that in the settings of the Fortnite game on whatever device you are using). They then transferred it from the account I had mistakenly  used. A hassle and I wish it could just be done on the device rather than having to go to the epic games website but support was awesome.
Rating: 3 stars