Make sure the game is closed. First you have to disable cloud saving. To do this go to the properties tab in steam

Then untick the cloud save at the bottom under the updates tab

The save location will be located here: C:\Program Files (x86)\Steam\userdata\xxxxxxxxx\368730 Where the x's represent a string of numbers, just delete all that is in there.

Reopen the game and you should be able to start a run from the press space screen