If you want to perform this kind of replacement, you will need to learn regular expressions (a.k.a. regexp).

There are dozens of tutorial on the net, e.g. here:
http://www.regular-expressions.info/

I should warn you though, that regexp are like women: powerful, mysterious and unforgiving :)
And they usually don’t behave the way you expect.
But don’t be discouraged. It is a great tool, if used properly. In my work, where I deal a lot with a plaintext processing, I would be lost without them

Anyway, answer to your particular question about space between two numbers is
with regexp search mode:
Replace
(\d),(\d)
with
\1 \2