if your main PC is a windows machine you need win32diskimager from sourceforge,net
in short you need 2 programs to install and run correctly a different OS:
a disk imager program (this is different program from an program that deals with ISO files (unless it specifically supports .img files)
a specialty unzip program that supports unzipping of the correct formats of.zip/.7z/.xz like 7zip
NOOBS IS an OS like RASPIAN or UBUNTU, you cannot install UBUNTU on NOOBS, it is not an application but an Operating System.
You need to download the ARM based .img file, NOT an .iso file. Ubuntu builds that are NOT specific to your Pi platform (ARM7) will not install.
Once downloaded (and being careful with full attention) extract the zip/xz package with 7zip or another supported unzip program into a directory of choice.
RUN the win32diskimager.exe being CAREFUL TO CHOOSE THE CORRECT SD CARD DRIVE, and then click to navigate to where the OS disk .img file is located and click WRITE, it may take some time to complete. Once it completes do not just unplug the SD card, EJECT it from within windows or linux or whatever OS your using to WRITE the .img file to SD.
***CHOOSING THE WRONG DRIVE TO WRITE TO CAN CAUSE YOUR MAIN OPPERATING SYSTEM TO BE OVERWRITTEN AND BE ERASED SO USE CAUTION WITH A DISK IMAGER SOFTWARE!!!!!
insert the SD card into your raspberry pi and power it up. (you may need to use the HDMI output as my official pi 7" touch has the screen upside down)