mv Celia\ Clarks\ Watercolor\ Works.html celiaclarkswatercolorworks.html
You need to escape blank characters on the shell or else it will be interpreted as an argument separator. Alternatively you can enclose the filename including blanks in quotes.
mv Celia Clarks Watercolor Works.html whatever
means "move the files Celia, Clarks, Watercolor and Works.html to the directory whatever".
To make things easier you can use tab completion: Just type
and then hit the TAB key. If the part of the filename you've entered is unambiguous it will be automatically completed.
You should read a beginners bash tutorial (google it).