I don't know ACAD, but my wife does. This is what she told me. Hope it helps, and that it is not too late.
"Use "break" command, either from "modify" menu (top of the screen, if this
option is used - from tools, options, display, display screen menu: checked), or
choose icon "break" or type "break" (alias "b" could work as well). The other
way would be to use "trim" command but that requires two lines intersecting the
line to be broken. F1 key starts Autocad help, which gives pretty good
instructions for each command."