This depends on how your install it.
If you have used the option, "install along side windows" You can simply delete is like any other software, If you install it separately. (You will mostly see GRUB boot loader loads first when computer starts)
In this case simply deleting ubuntu will be a huge mistake, You need to make windows boot loader primary. Then you can remove ubuntu. If you don't know much just leave the ubuntu as it is. And ask your friends help who knows it well.
You don't uninstall an Operating System the Same way you uninstall a program.
All you can do is format the Partiton Ubuntu is on.
Or replace it with something else.
by the way: Ubuntu is far better than Windows. More secure.
IF….you installed as a dual boot- boot up in windows- go to internet and download Easy BCD in windows and install it. Then open Easy BCD and follow instructions on screen to delete Linux and reclaim disk space…its as easy as 1-2-3.
IF….you installed Ubuntu over windows, and want windows back…..boot up on your ubuntu "install" disk/usb….once up and running….go to disk utility- delete Ubuntu partition- format partition to nfts or fat, whatever it was before-back out and shut down. Remove ubuntu disk- put in windows install disk and boot up…install windows.
if you installed ubuntu after windows it installs its own boot manager overwriting the windows one (even if its a dual boot system)
If you completely erase ubuntu you will need to use the windows recovery disc to reinstall the windows boot manager. – so make sure you got one