Assuming there is one (DCIM is used on devices with cameras), go to Computer and type DCIM into the search bar (top right).
If you mean how do you find it on your phone when the phone is connected to the computer, it's usually either /DCIM or /<whatever the internal or external SD card is/DCIM. (/ in a directory path means "start at the root or beginning", like C:\ for the C drive [Microsoft, in its infinite refusal to be normal, decided that \, not /, is "root".])