Answer

A user report can easily be edited but once it's turned into a user Reports but when Click on "Mark as System Report" users don't have a way to edit it.

This is how it works by design, once a user report gets to be marked as a System Report, it can’t be changed anymore. It just allows the NA user to move up, move down and delete the System Reports.

However, a workaround exists that can be implemented. Let's do the following:

There are 2 ways to edit it, by creating a test User report or use an existing one.

Note: Take into consideration to find out the filterID first in the URL of the system report needed.

For example: This is a test System report URL line where users can see the queryID 1611 in that case, so that is the ID to be used then.

https://16.40.163.58/saved.adhocquery.execute.do?queryId=1611&navAction=runSaved

Modify an existing non-system report , then replace the filterID with the system report filter ID and load the new URL, then Modify and save.
It will save the change in the system report.