This question is underspecified. Are you trying to reorder the 2D slices in a 3D volume, or the 3D volumes in a 4D time series? Why ais the image order scrambled - what is the source of these images and how were they converted (e.g. did you convert them from DICOM to NIfTI using dcm2niix)? I suspect a combination of fslroi and fslmerge can solve your issue, but it might be easier to fix it at the source.