Changing (flipping) line direction in QGIS?

If you want to reverse selected crossection lines in QGIS3.X, I recommend the plugin--digitizingtools. Of course it works well in qgis3.x. And at its tool bar, you can see flip line tool. Enjoy youself!

Tested on QGIS 2.18 and QGIS 3.4

I can suggest using a "Virtual Layer" through Layer > Add Layer > Add/Edit Virtual Layer...

Let's assume we have five features in "roads" layer accordingly, see image below.

With the following Query and using ST_Reverse, it is possible to achieve the result

SELECT id, Name, length, ST_Reverse(geometry)
FROM roads

The output Virtual Layer will maintain initial attributes and reverse geometries.

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Checking whether the query works proper possible with writing a small extra query

SELECT r1.id,
       st_astext(start_point(r1.geometry)) AS original, 
       st_astext(start_point(ST_reverse(r2.geometry))) AS reverse 
FROM roads AS r1
JOIN roads AS r2 ON r1.id = r2.id

The output table will prove that the original geometry was swapped for a new layer, see table below

Select line that you want to flip. Use field calculator -> go to last position -> reverse($geometry)

It's built into the app now and no plugins are required.

It is accessible via the Advanced digitizing panel that is not shown by default.

https://docs.qgis.org/3.10/en/docs/user_manual/working_with_vector/editing_geometry_attributes.html#reverse-line