There are two basic methods you can use to edit individual slices in Reason:
In the Waveform display of the Dr.Rex device.
This is used for making playback settings for a slice.
Here you can edit the notes that play the slices. There is a special REX lane for editing REX slice notes, with the notes indicated by the slice numbers instead of by pitches. Editing in the sequencer is described in the Sequencer chapter.