To turn off the Auto Sum feature in Microsoft Excel, you can follow these steps:

1. Click on the "Formulas" tab in the ribbon at the top of the Excel window.
2. In the "Function Library" group, click on the "AutoSum" dropdown button.
3. Click on "AutoSum" again to uncheck it and turn off the feature.

Alternatively, you can also use the keyboard shortcut "Alt + =" to toggle the Auto Sum feature on and off.

It's important to note that turning off Auto Sum will only affect the current worksheet. If you want to turn off Auto Sum for all new workbooks, you can go to the Excel Options menu, select "Formulas" in the left-hand menu, and uncheck the "AutoSum" box under the "Working with formulas" section.