$ sudo dpkg -i virtualbox-5.1_5.1.22-115126~Ubuntu~trusty_amd64.deb

dpkg: regarding virtualbox-5.1_5.1.22-115126~Ubuntu~trusty_amd64.deb containing virtualbox-5.1:
 virtualbox-5.1 conflicts with virtualbox
  virtualbox-5.0 provides virtualbox and is present and installed.

dpkg: error processing archive virtualbox-5.1_5.1.22-115126~Ubuntu~trusty_amd64.deb (--install):
 conflicting packages - not installing virtualbox-5.1
Errors were encountered while processing:
 virtualbox-5.1_5.1.22-115126~Ubuntu~trusty_amd64.deb

dpkg -l | grep virtualbox | grep ii

sudo dpkg -r virtualbox-5.0 # match the version of virtualbox with the results of step 1.

sudo dpkg -i virtualbox-5.1_5.1.22-115126~Ubuntu~trusty_amd64.deb