Hi,
Your notebook is listed as coming with a 640Gb HDD, but there's hardly any difference in cost if you replace it with a 750Gb drive - the HDD on the link below is an example of one that would be fine for your notebook.
The procedure to replace the Hard Drive starts on Page 51 of your Maintenance & Service Guide.
Once done, simply use your Recovery Discs/USB media to reinstall the Operating System on the new drive - full details can be found on the HP document Here.
If you don't have your Recovery Discs/USB media, you can order a replacement set using the link below.
If you have any problem with this link, see if you can order them directly from HP.
If you live in the US, contact HP Here.
If you are in another part of the world, start Here.
Regards,
DP-K