The file can be found in:
AppDataLocalLowCodename EntertainmentIdle Champions
to reach it, do the following:

1. Press Win+R keyboard shortcut to start Run
   
   
   
   
2. and Enter "."
   
   
   
3. To see AppData
   Make Sure you have hidden files/folders visible.
   
   
   
4. The file can be found in:
   AppDataLocalLowCodename EntertainmentIdle Champions
   

• Alternatively Enter %AppData% in Run
  to open AppDataRoaming and navigate back to
  AppDataLocalLowCodename EntertainmentIdle Champions

Open localSettings.json with Text Reader like:
Notepad or WordPad

Notes:

• All Changes to the File will require RESTART of the Game to bring them into effect!
  
• Opening the In-Game Menu after Changes
  can trigger a REVERT action for some changes.
  E.G. "UIScale":0.59 -> "UIScale":0.75

Resolution
is a variable that is based on each person's setup.
Using the localSettings.json you can modify it to fit your personal needs.

"resolution_x":1600,
"resolution_y":900,"

Fullscreen option
can be rendered irreversible in some cases using the In-game Settings Menu.
Using the localSettings.json you can modify it respectively:
false and true

"resolution_fullscreen":false

UI Scale
is locked at 75% Minimum and can't be lowered using the In-Game Menu.
The localSettings.json option is unrestricted.
Lowering the Value will make all Hero Slots visible depending on your resolution.

"UIScale":0.59

• UI Scale 75%
  
  
• UI Scale 59%
  

CAUTION: Going under certain level will make some texts
Unreadable or Disappear

Particles can be easily seen by observing the Dust Particles behind the moving enemy.
In-Game you can change the Particle Amount to
High, Medium and Low

The different options correspond to the localSettings.json
option value "PercentOfParticlesSpawned":


You can lower it even to 0 by

"PercentOfParticlesSpawned":0

Result:

• Particle Amount 0
  

The game have only Fullscreen and Windowed Options.
Using this method you can make it run in Borderless Mode.
It converts Windowed Mode to Borderless Mode.
This method can be used for other games as well.

To Apply:

1. Open Game Properties in Steam
   
   
   
2. Enter -popupwindow in Launch Options
   

Result:

1. Windowed
   
   
   
2. Borderless
   

Notes:

• Using Windowed for prolonged time can make your Monitor Pixels Stuck or Burn-In for a period of time even with LCD.
  
  
• Using Borderless will center your window in the middle of the screen and you will lose the ability to move it like with Windowed Mode.