Yes. As long as the appellate authority has not decided on your appeal, you may withdraw it.
Normally, your unit’s servicing JAG office will have you sign a memorandum for record that you voluntarily withdrew your appeal.
My concern would be, why are you withdrawing your appeal? The appellate authority can not make your punishment worse; the appellate authority can only leave the punishment the same or lessen it.
Disclaimer: I am not an attorney and any views presented are my own and are not to be
interpreted as legal advice. Furthermore, my views do not necessarily
represent the views of DoD or its Components.