An employee who receives sick pay is taxed on it. It is treated the same as other taxable compensation, which means it is taxable for income tax purposes and subject to FICA.
Employers who give sick pay to an employee can deduct the payment. Again, it is merely another form of taxable compensation.
On Form 1040X, you may file an amended return within a three-year period to claim a refund or correct a mistake made on an original or previously amended return.