If a notice of appeal is defective, the applicant will be advised of that fact in a letter. The corrected notice of appeal must be filed within 1 month of the mailing date of the letter or within the period for response set in the last Office action, whichever is later, to avoid dismissal of the appeal and, in some situations, abandonment of the application.
The position of the USPTO on this issue is described in the following section(s) of the Manual of Patent Examining Procedure (MPEP):
MPEP 1205 - Notice of Appeal
Law - Correction of Notice of Appeal