Generally you cannot take an OTA update while rooted. If you are running a custom ROM, you will want to flash an updated version of your ROM or another ROM.
If you are looking to use the stock os, you can download and flash the factory images to update and then install custom recovery again and flash the superSU binaries to add root again. Or you could use your custom recovery to flash a stock rooted 5.1.1 ROM if there is one available.