When did you sell this? 2017 or 2018? If you have not already put any entries on Form 6252, you don't do anything. Opting out means not filing the 6252. If you had one for 2017:
Revoking the election. Once made, the election can be revoked only with IRS approval. A revocation is retroactive. You won’t be allowed to revoke the election if either of the following applies.
One of the purposes is to avoid federal income tax.
The tax year in which any payment was received has closed.
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