I'm trying to modify a policy for a single system, the legacy has been cut and modified, but the changes apply to all servers, I need help.
what version of epo are you using?
can we have a screen grab of the breaking policy enforcement options screen you are seeing
If you are modifying the Policy Object then it will affect all the places it has been assigned does not matter whether you are modifying the the single system.
Create a duplicate of the policy with a different name. Make the changes you want. Then, assign the new policy to the single computer.