Depends on whether the laptop was designed to have the HDD replaced. Most older ones were.
They usually have an access-panel on the back, that you undo a couple of screws, remove the access panel, unscrew the caddy, pull out the caddy, unscrew the HDD from that caddy, and remove the HDD from the caddy.
Then you put the new HDD into the caddy, and put everything back together in the reverse order.
Then you have to use your restore discs, or an OS install disc, to re-install the OS.
Then you have to download and install drivers.
Then you have to do Windows updates.