Offer a substitute for it (since it does not absolve you of the responsibility of sacrifice). Thereupon he said: Messenger of Allah. 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 Messenger of Allah ﷺ said: Make it a substitute for that (and sacrifice it), but it will not suffice for anyone (as a sacrifice) after you. (Using translation from Muslim 1961i)
لَهُ رَسُولُ اللهِ ﷺ أَبْدِلْهَا فَقَالَ يَا رَسُولَ اللهِ لَيْسَ عِنْدِي إِلَّا جَذَعَةٌ وَأَظُنُّهُ قَدْ قَالَ خَيْرٌ مِنْ مُسِنَّةٍ فَقَالَ رَسُولُ اللهِ ﷺ اجْعَلْهَا مَكَانَهَا وَلَنْ تُجْزِئَ أَوْ تُوَفِّيَ عَنْ أَحَدٍ بَعْدَكَ