Optiplex 390: switch VGA to HDMI output

I have an Optiplex 390 SFF refurbished.in 2017/11.  It has an intel video card and both VGA and HDMI.  My monitor is a Lenovo Thinkvision 23" HiDef with sound and camera.  It has VGA and DisplayPort inputs.  So far I have used VGA because it was the only cable I had.  I now have a HDMI to Displayport cable, and want to use that for my monitor.  How do I get the Optiplex 390 to switch it's output to HDMI?  Every time the system restarts, it puts out video on VGA only, and the monitor tells me "no input signal" on DisplayPort.