I checked with the Dell website and Windows 8.1 doesn't seem to be supported by Dell for your machine. The drivers stop at Windows 8 although other users may have found a way to run 8.1 on that machine. It's just not supported driver wise unfortunately.
If you check this FAQ from Microsoft, under the question 'Can my PC run Windows 8.1' you'll find the 'Windows 8.1 upgrade assistant'. Try giving that a run and see what the outcome is: http://windows.microsoft.com/en-US/windows-8/upgrade-to-windows-8