Excel Macro to carry out 6 digit number recognition and arithmatic

I have a data set which sporadically includes 6 digit numbers that I need to pick out and replace with the result of some basic arithmetic.

sample data set

For the data set in question every time a 6 digit number appears I want to carry out the following:

cell value = n-65535 So an example -4 = 65531-65535 I would want to replace the 65531 that appears in the cell with a -4.