Confusion over radio button and terminology

I am designing a mobile application and was looking for some guidance on how to structure the inventory management section. This app allows for users to scan products and update the current Stock on Hand (SOH). From some initial testing I have found users are getting confused between the two radio buttons. See here for the example.

Mobile Application inventory management

Update stock means that the database will be updated to the value entered by the user. Add to stock means the current database value will be increased by the amount entered by the user (i.e. SOH 30 + entered value 5 = SOH 35).

Users are not really understanding the difference between the two. Did anyone have any suggestions to help the UX here for this?

Thanks,

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