A fillet arc is edited in the same way as other arcs, except the results of the edits affect the two objects from which it was created. This is because the fillet arc is dependent on the two objects.
Dependencies are relationships created between objects. If you edit an object, any other object dependent on it will update automatically. Certain edits break dependencies between objects.
To editing a fillet arc:
The fillet arc is highlighted and the graphic handles are displayed:
The two lines either side of it can be trimmed or extended to fit the fillet arc:
The arc and the other line update too:
The arc and the other line update too:
This breaks the dependencies and the lines go back to their original lengths:
This breaks the dependencies and the lines go back to their original lengths: