Factorio Forums

Right now, Factorio allows you to rotate entities, but not flip them. This means asymmetric entities, like refineries and chemical plants, can't be placed in some orientations. This also applies to many blueprints.

Adding a command that allows a flip or mirroring in addition to the rotation would allow for new varieties of design, and, in the case of underground belts in particular, could be used to allow you to rotate them like standard belts instead of simply swapping the direction back and forth, that functionality now being assigned to the mirror command.

Flip is not supported by Factorio, because train and refineries/chemistry cannot be mirrored in all cases.

There are several mods to flip.
https://mods.factorio.com/query/Flip?version=0.17

And I didn’t completely understand why you want to rotate underground belts.