Parsing/Replacing values in a string

I am having a brain cramp and can't seem to figure out the best way to do the following:

I have a string that is 34 characters long and filled with blanks.  I need to be able to replace characters at particular byte positions. For instance I need to be able to replace the characters at position 25 and 26 with 2 characters supplied by the user.  I can't seem to be able to get this to work properly.

Any suggestions would be appreciated.

Thanks

 

 
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