Instructions
Go to Sewer Run 2's start menu.
Enter the character menu by clicking "Character."
Locate the section of the character menu that says "Clothing." Scroll through the clothing menu until you have the stealth suit highlighted (the suit will have a red lock across it, indicating that it is not available).
With the suit still highlighted, click "Cancel" at the bottom of the character menu screen. This will take you back to the main menu.
Click "Play." Once the gameplay starts, your character will be clothed in the stealth suit.