Question: I have genuine windows 10 in my lappy. I tried to dual boot with Ubuntu but accidentally I replaced my windows with ubuntu. So now I have only ubuntu in my pc and I lost my genuine windows. How to get back my windows OS.I dont even have my back up image of my windows

Answer: 
If you really deleted the Windows partition, Ubuntu is not going to get it back for you. Sorry.

Not knowing if this computer originally came with Windows 10, I will make the assumption it's fairly new and did. You could post more info per <https://askubuntu.com/help/how-to-ask> :>)

When you start there should be a recovery partition. Look for the option to choose a boot device, perhaps F12, hold that key down and see if a recovery partition is a choice for bootup. Some computers hide this info at the start, and it's manufacturer specific, so you should consult your computer manual, again we don't know what model you have.

If you really overwrote everything then your best best is likely to contact the manufacturer. I am not aware that you can download windows 10 cold from the Internet. If for example you bought it from Best Buy or another retailer perhaps they can help.

This is probably a question for a more Windows oriented StackExchange site like <https://superuser.com/>

The only option that's relevant to askubuntu is that you could open a terminal window and run the command `fdisk -l` and see if there's still a recovery partition. If you are not familiar with Linux be very careful running fdisk, you should not need to use `sudo` to get this listing.
Download testdisk from : <http://www.cgsecurity.org/wiki/TestDisk_Download> unpack it, launch ubuntu from live, change to folder where testdisk was unpacked and run: 

```
sudo ./testdisk_static     

```

from program menu chose analyse and try to recover your partition 

This tutorial will help:  

<http://www.cgsecurity.org/wiki/TestDisk_Step_By_Step> 

It will be hard, or even not possible but worth to try.