1) Download the latest spell files:
http://www.project1999.org/files/eqchangesv27.zip
(if this post is out of date, just update "27" with the current number)
2) Unzip the eqchanges file into your base Everquest directory, overwriting any files it says.
3) Log on to P99 and play the game.
If you still cannot do step #3 after steps #1 and 2, you need to ensure you have the proper permissions in your base Everquest install directory to overwrite the files with the appropriately updated files.