You probably only need to pop the back off, make sure it's not full of grit and fluff, and put it back on again.
Just run your thumbnail around under the back bezel, starting from the middle of a long edge - knives or screwdrivers won't do the plastic any good. Try to avoid touching components and contacts once you have it open. Static won't do the components any good, and fingerprints aren't great on spring contacts.
The cover will easily clip back on with gentle pressure at the edges.