C drive full how to use D drive

Inspiron 3671  Just noticed the "C" drive is full but D drive is empty.  Not sure why the computer was not set up to use this without me (novice older user) having to try to figure it out. Seems like it should just work without my intervention.  How do I make the D drive part of C?   I tried "Delete Volume" on D Drive but then cannot "Extend Volume" on C drive as it is grayed out.  D drive is not right next to C on the diagram.  Other threads on this seem very old  and for other computers.  My computer is 5 years old and very usable except that my D drive seems to be a complete waste.  How do I use it?