If you mean on the internal screen, and it shows 1920X1080, you cannot -- at least not short of replacing the FHD display assembly with a UHD screen assembly. If it's on an external monitor you're trying to get 4K, you'll need to check your monitor specifications to see what the external display supports.  
Does anyone have any idea on how to enable 4k on this model?  Thanks
XPS 13 9310 2-in-1, can only select 1920x1080