Online to Download

I have just been put in charge of the company website which is all based on WordPress with a custom theme. It appears as though this has only ever been managed online and has not been upgraded from 2.9.2 yet.

As I will be implementing a new layout and style I would like to be able to work on the site, making the changes as required, offline without effecting the site as it stands online until I have everything ready.

I am very new to WordPress and I just want to know if I download WordPress will I be able to play around with the site locally without changing what is currently online, then update the actual site when I have everything as I want it?