If an appeal brief is defective, the appellant 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 allowed for filing of the appeal brief, whichever is later, to avoid dismissal of the appeal and, in some cases, 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 1206 - Appeal Brief
Law - Correction of Appeal Brief