Some dock icons changed to white after update to El Capitan
After updating to El Capitan on my iMac (mid-2010), two of my icons that are on my dock (Messages & Notes) changed to white sheets of paper with a folded tip at the right uppermost corner and a little black arrow at the left bottom corner. Is there a way that these icons can be changed back to the more conventional pictures that existed previously. Please provide detailed steps or link to same. Thanks.
iMac, Mac OS X (10.7.1)