Unreal Engine 4.x Scripting with C++ Cookbook - Second Edition

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Quaternions, being vectors themselves, can be rotated. Simply extract the (x, y, z) components of the quaternion, normalize, and then rotate that vector. Construct a new quaternion from that new unit vector with the desired angle of rotation.

Multiplying quaternions together represents a series of rotations that happen subsequently. For example, a rotation of 45º about the X-axis, followed by a rotation of 45º about the Y-axis will be composed by the following:

FQuat( FVector( 1, 0, 0 ), PI/4.f ) * 
FQuat( FVector( 0, 1, 0 ), PI/4.f ); 

This would give you a result that would look similar to this: