If your game is launching in a different resolution than your monitor/tv (for example 3840 × 2160 instead of 1920 x 1080) and is stuck on said resolution you can fix this by going to appdata, finding sekiro and changing the screen size in the graphicsconfig.xml file with for example notepad++.

the place you want to change is this:
<Resolution-FullScreenWidth>[change_to_monitor_width]</Resolution-FullScreenWidth>
<Resolution-FullScreenHeight>[change_to_monitor_height]</Resolution-FullScreenHeight>

hope this helped :)