REPLACEB( Text ; Position ; Length ; NewText )
Text: A text expression or reference to a cell containing a text expression in which the bytes are to be replaced.
Position: the byte position from which text is to be replaced.
Length: the number of bytes to be replaced.
NewText: the text to be inserted.
=REPLACEB("ᄩᄔᄕᄜᄝᄞᄠᄢᄣᄫᄬᄭᄮᄯᄲᄶ";4;1;"ab") returns "ᄩ abᄕᄜᄝᄞᄠᄢᄣᄫᄬᄭᄮᄯᄲᄶ" .
Refer to the REPLACEB wiki page for more details about this function.