TLDR: If you wish to correct a declaration that has already been signed, you must correct the information in the previous declaration and sign it again. However, the available sources specify neither the form, nor the service, nor the supporting documents to use specifically for a foreign crypto account declaration.
Correcting after signing
The general rule states that you can correct the information entered in the previous declaration and then sign it again. This rule is formulated for declarations in general and does not describe a specific procedure for foreign crypto accounts.
Complying with the applicable deadlines
Corrections must be made within the “additional deadlines” provided for the declaration concerned. The available sources do not make it possible to determine their duration for a foreign crypto account declaration.
For a voluntary amended declaration, the filing must take place before the expiry of the period during which the administration may exercise its right of reassessment. This rule is presented within a general tax framework, and no specific deadline for foreign crypto accounts is confirmed here.
If an audit has already begun
For a correction made after an audit has been opened, the source refers to BOI-CF-IOR-20. The available excerpt does not describe the procedure to follow in this situation.
Information that the sources do not make it possible to confirm
You cannot determine from the available sources which French form to use, or whether the correction must be made through an online service or a paper procedure.
The sources also do not specify which documents or supporting evidence should be attached. They do not establish a specific penalty applicable to an error or a late correction concerning a foreign crypto account declaration.