Open the Software Manager and type in NASM in the search box, hit return, when the list is filled, double click on nasm, then click the button that says install.
You can also install it with the Package Manager, in the quick filter edit control, type in nasm, hit enter and click the box next to nasm, select choose for installation, then the apply check mark at the top...
Or from the terminal - sudo apt-get install nasm.