How can i backup my Lemmy account for transferring it to other instances

You can't, but there's a github issue to enable this functionality: https://github.com/LemmyNet/lemmy/issues/1985

FYI someone made a script to copy user account communities from one instance to another : https://sh.itjust.works/post/168439

I'm not entirely sure if that's possible right now. There are many requests on the github page for such a feature, but the devs have understandably other priorities.