Before you can wipe save data, you need to disable steam cloud save. Otherwise, the game will re download a saved copy of your good save every time you launch the game. If you already have steam cloud save disabled, good job! You are ahead of the curve.

Right click on Death Must Die in your steam library.

Select Properties

Uncheck the box labeled "Keep game saves in the steam cloud for Death Must Die"

Congrats. Your game is no longer being backed up to the steam cloud. FYI, a copy is still saved, so if you ever want your save back from this point you can either make a physical backup in the next step, or you can simply re-check this box and pick the cloud save when it asks which version you want.

Your local save is named "Slot_0.sav" and can be found (probably) at
C:\Users\*PCUSERNAMEHERE*\AppData\LocalLow\Realm Archive\Death Must Die\Saves

If you want to back up your save, you can copy this file somewhere safe.

If you don't want a copy of your save, or you trust the steam cloud, you can just delete this file.

If you can't find the save file there, you can always search for it by name. I've always been a fan of voidtools "Everything" search tool. It's super fast and very helpful. It can be used to search the entire PC much faster than file explorer can. If the save file isn't found at the above location, you can try searching using either file explorer or Everything for "Slot_0.sav" or for "Death Must Die" and then finding the right folder from there.

This is the most technically demanding stage, but what I need you to do is launch the game again from steam. Select Play.

...

That's it! You should start as our favorite knight, ready to face the deathly denizens of...Death completely stripped bare of signs, upgrades, or companions.

Enjoy the climb!