Macro Find and Replace Code

I need an easy macro code that will occur AUTOMATICALLY when I open the file. I need it to search in column B and replace a 3 digit number (example "118") to a four digit number (example "1118"). I have about 100 different numbers, like 121 and 184, etc., that indicate a particular item, and the code has changed to a 4 digit number by adding a "1" in front. I need to reflect this in my spreadsheet. Can anyone help me? I feel that it should be fairly simple code, but I've never used macros before.

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