If after receiving your income tax avis d’imposition this year you believe you made an error in your declaration, then you have until November 25 to make a correction if you declared online. You can do this by clicking Corriger ma déclaration in your personal space at impots.gouv.fr A new avis should be sent a few weeks later.
There will be no penalties for late declaration if you respect the time limits.
If you declared by paper, you must make a réclamation, either by writing to the tax office, preferably by recorded delivery, including a copy of the income tax avis or via your personal space under Nous contacter > Ma messagerie sécurisée.