First thing is to navigate to your Darkeden New Blood folder. Open the folder and look for "Data" folder. Should be very first folder. Right click on it and open properties. Towards the bottom uncheck the box that says "Read Only" a pop up will appear, leave it as "Apply to all sub-files and sub-folders" and press ok. Now we can edit the resolution.inf file.
Since we are already in the Darkeden New Blood folder just go to Data -> Info and open up resolution.inf. Now just set your res to whatever you like. To change your game to be in windowed mode, change FullScreen to 0, if you want to keep it full screen, leave it to 1. Remember, you need to close your game before you change them, otherwise it's going to change it back to the resolution you have while your game is opened.
Recommended Resolutions:
4:3: 640×480, 800×600, 1024×768, 1152×864, 1280×960, 1400×1050, 1600×1200, 2048×1536.
16:9: 640×360, 854×480, 960×540, 1024×576, 1280×720, 1366×768, 1600×900, 1920×1080.
16:10: 1280×800, 1440×900, 1680×1050, 1920×1200.
If you get an access denied error when trying to save the res.inf file. What you have to do is change your access control to the res.inf file. You can only read the file by default but can't save any changes to the file.
Navigate to the resolution.inf file. C:\Program Files (x86)\Darkeden United V6.67\DK United V6.67 Client\data\info. (Make sure to show hidden files.) Right click on the res.inf file and click properties. Navigate to the security tab and click the edit button. Find your user name, click it and then check the box for "Full Control". Click Apply. You will now be able to save the res.inf file.