=SUBSTITUTE(OriginalText, TextToRemove, TextToInsert, instance_num)
OriginalText → The Text or the reference to a cell containing text for which you want to substitute characters or text string.
TextToRemove → The text to be replaced with TextToInsert.
TextToInsert → The New Text - The Text you want instead of TextToRemove.
instance_num→ (It is Optional) - It specifies which occurence of TextToRemove you want to substitute with TextToInsert. If you specify instance_num, then only that instance of TextToRemove will be replaced. If it is omitted all instances of TextToRemove will be substituted with TextToInsert.
THINGS TO BE TAKEN CARE OF:-
1. This formula is case-sensitive. So be aware of the usage of UPPER, LOWER and PROPER Function also as your case may be.
2. Omitting instance_num will substitutes all the instances of the given Text or Cell Reference.
Basic Use of Substitute Function
View Advance use of Substitute Function on Microsoft Website:-
1 Formula to count the number of occurrences of a text string in a range
2 Formula to count the number of occurrences of a single character in one cell
3 Formula to count the number of occurrences of a single character in a range
4 Formula to count the number of words separated by a character in a cell
View the description of these examples of SUBSTITUTE function on Microsoft Website - http://support.microsoft.com/kb/187667