These are my notes for updating my Witness on Ubuntu 14.04. These instructions assume that you have already completed a full installation previously.
First you must cd into your base steem directory. Then, these commands should get you there (The dot at the end of the cmake command is required):
git pull origin master
git submodule update --init --recursive
cmake -DENABLE_CONTENT_PATCHING=OFF .
make
The make process should take a while. If your machine has multiple processors, you can speed it up by instead specifying the number of processors (4 for example) in the make command:
make -j 4
Now, perform the following commands:
cd programs/steemd
sudo ./steemd --replay-blockchain --rpc-endpoint
At this point the database will rebuild and all HARDFORKS will be applied. Then your miner will be up and running again. Enjoy.