REPLACE

Replaces characters in a text string beginning at the offset position.  The first character in the text string is always in offset position 1, the second is 2 and so forth.

Syntax

REPLACE(text-expression,start-offset,num-chars,new-text)

Arguments

text-expression Text or cell reference to evaluate.
start-offset Starting character position where the replacement begins.
num-chars Total number of characters in the text to replace.
new-text The text to insert. 

Example

REPLACE("Go be there.",3,2,"sit") returns "Go sit there."