MS Access update query not updating errors out

I have a query in Access that I started as a Select Query. I am getting the correct results and am trying to update a specific field in a table with the results. How do I do that? This is an elapsed time from when someone took possession of a room in a house to current date. The calculated field works but for some reason whenever I try to change it from select to update it errors out.

Field = Days: DateDiff("d",[entrydate],Now())
Update to=[Rates].[TotalDays]

Error=DateDiff("d",[entrydate],Now()) is not a valid name. Make sure it does not include invalid characters or punctuation and that the name is not too long.

Any help is greatly appreciated.