How to merge two labels in an labeled image

I have an image with three or four labels and i want to merge two of them and label it as a single label. How to do it? Convert 3D GUI is not helping me regarding this. Any way through ITK Snap or slicer or ITK modules available to do this?

I suspect the ChangeLabelImageFilter is probably what you want.

You could also do it through Slicer. Import the label map into Segmentations module, edit the segment, apply logical operation Add, remove the label you just “added”, and export to label map again.