insert decimal point after first digit

I have a cell with many digits in it, ie 12345.
I wish to change it to a 1,2345. insert decimal point after first digit after all my digits
Is there a macro command that will allow me to select all cells in range. I have seen previous threads in this forum, however when I insert my formulas, I get an error message. My formulas seems to be corrupt in some way.
kind regards
Henrik

Thanks. Realy helpful. Have a nice day:-)