How to change hard disk of my laptop

@mohammedrakeen iFixit does not have a guide for this yet. You selected the Acer Extensa 4620 as your device, you need to:

1. Unplug your Acer Extensa 4620's power cord and all other peripheral cables (keyboard, mouse, monitor, etc).
2. Open up the back panel of the laptop and locate the hard drive. It should be mounted in a special tray and should be the largest component in the laptop.
3. Find the hard drive's SATA cable and disconnect it from the motherboard.
4. Unscrew the screws that are keeping the hard drive in place and remove it from its tray.
5. Place the tray aside and replace the hard drive with a new one.
6. Reconnect all cables and close up the back panel.
7. Plug the power cord back in and boot up your laptop. from here

If you prefer a video, use this one The very first minute shows you how to replace the HDD

While you work on your Laptop, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.