Hi Kiet 

It is not recommended to downgrade an Odoo module from version 16 to version 12, as there may be significant changes in the code and database structure between the two versions that could cause compatibility issues.

If you have upgraded your database to Odoo version 16 and have data in it, downgrading it to version 12 could result in data loss or corruption. Additionally, modules that are compatible with version 16 may not be compatible with version 12.

However, if you want to try to downgrade a module, you can try the following steps:

1. Export the data from the module in version 16 using Odoo's built-in export functionality.

2. Uninstall the module from version 16.

3. Install the version 12 of the module.

4. Import the data into the version 12 module using Odoo's import functionality.

Please note that this process may not work for all modules, and it is highly recommended to test it on a development or testing environment before attempting it on a production database. It is also recommended to consult with a qualified Odoo consultant or developer before attempting to downgrade a module.