The Excel Replace function is similar to the Excel Substitute Function. The difference between the two functions is:
The Excel Replace function replaces all or part of a text string with another string.
The syntax of the function is:
Where the function arguments are:
old_text    The original text string, that you want to replace a part of. 
start_num    The position, within the old_text string, of the first character that you want to replace. 
num_chars    The number of characters to replace. 
new_text    The replacement text. 
Note that the Excel Replace Function is not suitable for languages that use the doublebyte character set (eg. Chinese, Japanese, Korean). These languages should use the ReplaceB function, which is explained on the Microsoft Office website.
The following spreadsheets show two examples of use of the Excel Replace Function.
Formulas:
 Results:

Further information and examples of the Excel Replace function can be found on the Microsoft Office website.
If you get an error from the Excel Replace function, this is likely to be the #VALUE! error:
#VALUE!    Occurs if the supplied start_num or the supplied num_chars arguments are negative or are nonnumeric values. 
The Excel Replace function is designed for use with text strings and returns a text string. Therefore, if you attempt to use the replace function with a date, time or a number, you may get you 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:
The Replace function should now work as expected on the values that have been converted to text.