On Windows:

• Press win-r to summon Run window
  
• Paste the following into the Run window
  
  %LOCALAPPDATA%/VaultoftheVoid
  
  
• Press OK and you should be greeted with Vault save folder
  
• Find
  PlayerProgress.json
  and open it with your favourite text editor (or Notepad should work well enough)

On macOS:

• Select Finder, and then press command-shift-g to summon Go To... prompt
  
• Paste the following into the Go To... prompt
  
  ~/Library/Application Support/com.vaultofthevoid
  
  
• Press enter and you should be greeted with Vault save folder
  
• Find
  PlayerProgress.json
  and open it with your favourite text editor (or TextEdit.app should work well enough)

Make sure to back up the PlayerProgress.json file! Also you may want to turn off Steam Cloud temporarily so your edits won't be mysteriously overwritten by an existing save file.

Inside the PlayerProgress.json file, find The default value of max instability should be 70, feel free to increase to a number that feels like enough pain for you. For reference, current max instability as of version 2.5.22 is 155, but in the upcoming 2.6.0 it'll be increased to 160.


Save the file and boot up the game, and after finishing a run at I+70 you should see your instability cap raises again. This value does NOT change your current instability cap but only stops the game from raising instability cap further.

If you would like to skip the grind from finishing runs at current Instability cap to raise it further, search in the PlayerProgress.json for The number after this should be your current instability cap. Change to 160 and save and you'll never need to grind instability cap again.
However I highly recommend not to go into a run with everything turned on just after you've won your first I+70 run, and I'm not responsible for sudden death at first elite on floor 1. Increase it by 5 to 10 at a time and adapt to your pain threshold gradually.

Remember to turn on your Steam Cloud if you previously turned it off. And we would like to be clear that I+70 is the "developer intended hardest difficulty that is still reasonable and fair" and there's no achievement above I+50, so it's totally up to you to increase the challenge and have fun (or pain).