How do I edit non-financial asumptions?

Non-financial categories are edited in the same place, and in the same way as your financial data. The only difference is that when they were created they were flagged as being non-financial, so don't appear on your P+L dashboards.

On the Data Sheet, scroll down through the Category drop down on the top left, and you'll go past your Revenues, COGS, and Expenses, and if you keep going, you will see all your non financial assumptions. 

Select the one you need, and start editing.