The Excel Substitute function is similar to the Excel Replace Function. The difference between the two functions is:
The Excel Substitute function replaces one or more instances of a given text string, within an original text string.
The syntax of the function is:
Where the function arguments are:
text    The original text string containing the text to be replaced. 
old_text    The substring that is to be found and replaced by new_text. 
new_text    The new text string that is to be used to replace the old_text. 
[instance_num]   
An optional argument which specifies which occurrence of old_text should be replaced by the new_text.

Note that the Excel Substitute function is case sensitive. Therefore, if the old_text argument is the text string "A", this will NOT replace instances of the lower case text string "a".
Column B of the spreadsheet below shows four examples of the Excel Substitute Function.
Formulas:

Results:

For further details and examples of the Excel Substitute function, see the Microsoft Office website.
The Excel Substitute function is designed for use with text strings and returns a text string. Therefore, if you attempt to use the substitute function with a date, time or a number, it may return unexpected results.
If you are not planning to use the date, time or number in further calculations, you could solve this problem by converting these values into text, using the Excel Text To Columns tool. To do this:
The Substitute function should now work as expected on the values that have been converted to text.