Hi,
Most likely you can just replace the button, if need to change the whole keyboard, it is a very simple process if you are able to get hold of a replacement keyboard (be it new or old).
To change the Dell notebook keyboard: turn the notebook around, you will see quite a number of screws with a "k" next to it. Un-screw all the screws with the mark "k" will release the keyboard, the keyboard is linked to the computer board by a Ribbon cable.
If you want cheapest solution, go to SLS, look for those shops behinds with alots of very old notebook, you might be able to find one old keyborad from the computer. Ask around, some shops (those computer/noteboon repair shop) may have keyboard alone in stock.
But first check with Dell, how much is the replacement of the button or the whole keyboard before you decide on the DIY repair.