Use the :w
command with a filename:
:w other_filename
The following command will create a copy in a new window. So you can continue see both original file and the new file.
:w {newfilename} | sp #
Thanks for the answers. Now I know that there are two ways of "SAVE AS" in Vim.
Assumed that I'm editing hello.txt.
After save new file press
Ctrl-6
This is shortcut to alternate file
Source: Stackoverflow.com