I know to fix desktop pc's but in laptop I'm not really familiar. I want to replace the hdd of my laptop. This is an old laptop but I know I can still use it. But I don't know how to open it. My laptop is NEC versapro va75h

Hmm, usually there are several access panels on the bottom of a laptop. One of them should be the HDD bay.

Another panel could be for the battery, another one for the RAM slot, another panel to clean the CPU fan, etc.

There's no access panel for the hdd. Only for the ram and the battery. In nec versapro va75h, do I have to unscrew the bottom part? On the top? Or on the keyboard?

Unless someone else has more info, your on your own I'm afraid. Or you might try here (http://www.sparesweb.com/Spares/NEC.htm) for help.

See If there are screws marked with arrows. Remove these.
May have to remove any covers for Ram and battery, Remove RAM and Battery,
Look for screws.
Above Link by HVAC888 looks like may have your info. All Else, May Call Them.