Welcome to the Local Guides Connect community, @Mohan.V!
Your last sentence “I am using a Windows 10 PC.” gives the clue:
On the desktop, i.e. using Google Maps in a Web browser, there is no way to add a caption (as of my knowledge).
Using the Mobile app, there are three ways:
- Using “My Contributions”, tapping on a photo and then the pencil icon allows you to add a caption. Doing it this way has the advantage, users can see the caption at the POI’s photo listing, but note, we do not get any points for doing so (nevertheless this is my favorite way to add a caption whenever it makes sense.)
- Using the Update tap. This is described in the link provided by @new.user as well as here and in this discussion.
- By adding names to food dishes.
If you upload photos from the desktop (what I always do), you can add captions afterwards from a mobil, using either the first or third method described above.
The second method can only be used by uploading photo and caption at the same time from the mobile app,
Hope, this helps.