These values are the elements of the direction matrix of the volume based on which it is placed in the RAS coordinate systems (from IJK which is the voxel space of the volume). You cannot change it manually because:

This matrix is a component of the whole IJK2RAS matrix, which also contains origin and scaling

Editing manually a direction matrix is not a good idea because that way you couldn’t guarantee a valid matrix (orthogonal axes, etc)

The easiest way to modify the volume directions is applying a transformation on top of it, for which see the Transforms module.