Settings such as HDMI video output are configured in the /boot/config.txt file – you can edit this on the Pi with ‘sudo nano /boot/config.txt’. If you need to edit on another machine, see details here.
This page lists available video modes. On my LG E2341 connected by HDMI, uncommenting and changing these two settings works to set the output to 1080p:
hdmi_group=1
hdmi_mode=16