26 CFR 1.1402(a)-16: (a)-16 Exercise of option.

A taxpayer shall, for each taxable year with respect to which he is eligible to use the optional method described in § 1.1402(a)-14 or § 1.1402(a)-15, make a determination as to whether his net earnings from self-employment are to be computed in accordance with such method. If the taxpayer elects the optional method for a taxable year, he shall signify such election by computing net earnings from self-employment under the optional method as set forth in Schedule F (Form 1040) of the income tax return filed by the taxpayer for such taxable year. If the optional method is not elected at the time of the filing of the return for a taxable year with respect to which the taxpayer is eligible to elect such optional method, such method may be elected on an amended return (or on such other form as may be prescribed for such use) filed within the period prescribed by section 6501 and the regulations thereunder for the assessment of the tax for such taxable year. If the optional method is elected on a return for a taxable year, the taxpayer may revoke such election by filing an amended return (or such other form as may be prescribed for such use) for the taxable year within the period prescribed by section 6501 and the regulations thereunder for the assessment of the tax for such taxable year. If the taxpayer is deceased or unable to make an election, the person designated in section 6012(b) and the regulations thereunder may, within the period prescribed in this section elect the optional method for any taxable year with respect to which the taxpayer is eligible to use the optional method and revoke an election previously made by or for the taxpayer.