Please follow the following steps to change, modify tax nambers.

• Go to "Configuration"/"Company management"

• Go to "Taxes"

• Click on the next available line (as you are about to add new tax)

• In the "Management of tax groups" window, you can modify the tax number and the GL code.
• Save and exit.

Note: In "Tax group Management" the change will not be visible right away, you have to restart the "Company maintenance" module.