@arisha
A warm welcome to Mac-Forums.
What Patrick says is important. All Microsoft Office apps need to quitted for the installer to work. (ie not just Word)
And to reinforce Patrick's excellent advice, "closing" is not the same as "quitting".
So, it may well be that you have closed Word or any other Office app, but they all need to be quitted.
Patrick has told you how to Quit these apps. You could also use the Command plus Q keyboard shortcut; or click on the app, say Word, in the top menu bar and choose Quit from the drop down menu.
Ian