Replaces a piece of the specified text with new text.
replaceb( old_text, start_num, num_chars, new_text )
old_text: (Text) The original text before replacement.
start_num: (Number) The positional index in bytes of the first byte of the substring to be replaced.
num_chars: (Number) The length in bytes of the substring to be replaced.
new_text: (Text) The text that will be inserted into the old text in place of the extricated substring.
Text
The first byte of the entire text value is considered to have an index = 1.
replaceb("oldtext",1,3,"new")
returns newtext