If a taxpayer has found an error on their originally filed 941 Employer’s Quarterly Federal Tax Return, then that taxpayer should "make the correction using Form 941-X, Adjusted Employer’s Federal Tax Return or Claim for Refund."
Form 941-X is filed separately from Form 941. For more information, Taxpayers can visit the IRS website at Internal Revenue Service
and should enter the keywords Correcting Employment Taxes. Alternatively, taxpayers can consult section 13 of Pub. 15 (Circular E) for more information.