First, if the DVR was working at the default resolution (1280x720) and it was manually set too high, hold the reset button on the back of the DVR for 30 seconds to bring it back to default resolution (1280x720). 

***WARNING: This will reset all the other settings back to default as well.***

Walkthrough to Lower Resolution for ZMD-DX-SAN8, ZMD-DX-SIN8, and DS-H81A

Power off DVR, wait 5 seconds

Power DVR back on

Wait until loading screen goes away

Wait another 5 seconds for it to finish loading.

For the ZMD-DX-SAN8 or DS-H81A, you will use Zsight app remote control option.

Zsight Remote Instructions: http://kb.zmodo.com/index.php?solution_id=1329

For the ZMD-DX-SIN8, you can use the buttons on the front panel of the DVR.

Press the following buttons in order:  

Menu

OK x 7

Esc

Down

Ok

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(only do these steps if DVR was Reset or it is the first time hooking up and the Start-Up Guide has not been disabled yet)

Esc x 2

Menu

Down

OK

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Ok

Down x 5

Ok

Right

Down

Ok

Up

Ok

Down

Down

Ok

Up

OK x 7

Esc

Down

Ok

DVR will reboot with the lower resolution (1024x768)