Hi Guillaume
I am assuming you are modifying an ACL which is applied on an interface via which you are connecting to the device.
Few questions ..How are performing this operation ? Is it
1. Via CLI - in which you could
remove the use of the ACL
remove the ACL,
Downlad the modified ACL
restore the use of the ACL.
2. If you are using ACL Manager tool, you can choose to download the config using TFTP option. This will ensure you retain connectivity.
Let me know if you need more details.
Thanks,
~preetham