I need to modify a WorkFlow within a project, which is related to 5 other Projects. My request is: how can I modify the Workflow within a project without affecting the remaining 5 Workflows?
Workflows do not exist "in" projects - they are separate items held globally, and referenced by projects when needed.
If you change a workflow, it's going to affect all the projects using it because there is only one workflow that you're changing.
If you want to change a workflow a project is using, you can, but it will affect all the projects using that workflow. To make a project work differently to others, you need to tell it to use a different workflow (in your case, probably by copying the current workflow and using the copy instead)
You should copy the shared workflow and then make your changes to the new inactive workflow.
Then you will probably want to copy the shared Workflow Scheme as well and change out the previous shared workflow for the new workflow you just created.
Then attach the newly create Workflow Scheme to the project that needs the new one.