Changing (flipping) line direction in QGIS

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

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

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

Let's assume we have five features in the 'roads' layer accordingly, see the 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

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!