Use Kaldaiens FIX Souls "Unsqueezed" from here: https://steamcommunity.com/app/374320/discussions/0/357284131807728012/ files available from the PC Gaming Wiki Community Files (search there).
Quick-Start Guide:
Extract the Files --> ...\steamapps\common\DARK SOULS III\Game
Run ds3t.exe
Select the Graphics tab and set Aspect Ratio Correction to On
Select your desired resolution from Display Properties
For a detailed description of some of the more technical options in ds3t.exe, refer to the SoulsUnsqueezed.ini and dxgi.ini sections of the guide.
=====
Alternative Hex edit:
Hex edit: Go to --> ...\steamapps\common\DARK SOULS III\Game\DarkSoulsIII.exe
Look for 800700003804 and change it to 000A00003804 (2560x1080) or to 700D0000A005 (3440x1440)
INI edit: Go to --> C:\Users\...\AppData\Roaming\DarkSoulsIII\GraphicsConfig.xml
Change to this Values for 2560x1080
<config><ScreenMode>WINDOW</ScreenMode>
<Resolution-WindowScreenWidth>2560</Resolution-WindowScreenWidth>
<Resolution-WindowScreenHeight>1080</Resolution-WindowScreenHeight>
<Resolution-FullScreenWidth>2560</Resolution-FullScreenWidth>
<Resolution-FullScreenHeight>1080</Resolution-FullScreenHeight>
or Change to this Values for 3440x1440
<config><ScreenMode>WINDOW</ScreenMode>
<Resolution-WindowScreenWidth>3440</Resolution-WindowScreenWidth>
<Resolution-WindowScreenHeight>1440</Resolution-WindowScreenHeight>
<Resolution-FullScreenWidth>3440</Resolution-FullScreenWidth>
<Resolution-FullScreenHeight>1440</Resolution-FullScreenHeight>
Make the File under Properties READ-ONLY
Start the Game and in the Menu press ALT + RETURN (makes the Game Fullscreen and HOR+)