Offer a substitute for it (since it does not absolve you of the responsibility of sacrifice). Thereupon he said: Allah's Messenger. I have nothing with me but a goat of less than six months. Shu'ba (one of the narrators) said: I think he (al-Bara' b. 'Azib also) said: And it is better than a goat of one year. Thereupon Allah's Messenger (ﷺ) said: Make it a substitute for that (and sacrifice it), but it will not suffice for anyone (as a sacrifice) after you.
Sahih Muslim Book 35, Hadith 14
← Previous Hadith Allah's Messenger (ﷺ) addressed us on the day of Nahr and said: None should sacrifice the animal unless he has completed the ('Id) prayer. A person said: I have a milch goat of less than one year, better than two fat goats. Thereupon he said: Sacrifice it, and no goat of less than a year of age will be accepted as sacrifice after you.

Sahih Muslim Book 35, Hadith 13 Next Hadith → This hadith has been narrated on the authority of Shu'ba with the same chain of transmitters, but did not mention tht doubt (expressed in his statement) That is (the goat of less than a year) is better than a goat of more than one year.

Sahih Muslim Book 35, Hadith 15