Re: Emacs: adding 1 to every number made of 2 digits inside a marked region.

[Gregory Heytings]

> But how to increase/decrease in steps that are not equal to 1?

And this is exactly the point of macros: you can easily define another 
macro with the parameters you now need.  E.g. if you want to multiply the 
numbers by 2 you just have to change

(insert (format "%d" (1+ (number-at-point))))

into

(insert (format "%d" (* 2 (number-at-point))))

Of course, if this is something you regularly need, it's better to use a 
defun, or to record the macro in your init file by copy-pasting the result 
of M-x insert-kbd-macro.