I installed the Networks plugin in QGIS 3.4.7 successfully, but it does not appear in the Vector Tab.
Can anyone suggest another way to reverse the direction of a vector used in a network (graph)?
How to install and use a QGIS plugin
But seriously...The Networks plugin adds a new section to the Processing Toolbox:
This is a weird place for a plugin interface. They really should mention that in the plugin description.
As shown in Answer 1 the Networks shortcut plugin really is in the Processing Toolbox, with many tools including Reverse Arcs. But the Reverse Arcs tool reverses the direction of the entire network.
REVERSE ARC DIRECTION TOOL
This tool, found in Processing Toolbox, reverses the entire network or reverses just the selected arcs, always creating a new layer and a new file. If someone chooses to revert only a few selected arcs, the output file will include just these in the new layer and in the new file, this can be cumbersome for large networks.
SWAP VECTOR DIRECTION PLUGIN
The Vector Direction Swap plugin seems to be a good alternative to reverse the direction of only the selected vectors. This plugin does not create a new layer or file, it edits the active layer itself. The shortcut for this plugin can be found in the Vector tab.
To reverse some network arcs: enable the layer, select the arcs of interest, click Reverse Direction of Geometry. Then save your changes and close the vector editing.