Substitute is not intended for that use. If you were to look it up in Help, you'd find that it is intended to take text found in one field and replace it with different text. It can take "This is a test" and replace "test" with "job" to produce "this is a job".
Set field can move data from the field to a repetition in a repeating field.
But I advise against using repeating fields! They were useful back in FileMaker 2.5 when FileMaker was still a flat file system, but since FileMaker 3, using a related table of records with one record in place of each repetition is almost always the better, more flexible option.
I recommend setting up a related table and using a portal (Or row of one row portals) on your layout in place of the repeating field.