This is the data file and tools to Patch World of Warcraft from v4.1.0a to v4.2. This .zip includes the data file wow-update-base-14480.MPQ and several tools.
1\ extract all files and place everything except the MPQ in your main WoW folder
2\ place wow-update-base-14480.MPQ into the \Data\ subdir of your WoW folder
3\ run the launcher to patch.
World of Warcraft Client Patch 4.2.0a
Using an instant cast spell right after a target dies should no longer re-target the dead target's body.
Using an ability immediately after switching targets should no longer affect the previous target.
Resolved a movement error when jumping onto an object while experiencing a low frame rate.
Mac: Fixed a bug preventing shift and mouse wheel commands from registering properly.
Resolved issues that were causing client crashes near Beth'tilac in the Firelands.
Disconnecting while riding as a passenger in a vehicle should no longer cause an error upon subsequent login.
Entering a password with Japanese Hiragana selected as the keyboard type should no longer cause a crash.
Resolved an issue that was causing the client to crash under certain conditions when minimizing the game client.
The tooltip color for heirloom items has been returned to gold.
A move enforcement issue has been resolved that was affecting the death knight starting area quest N Death Knight  Death Comes From On High.
Tarecgosa's platform during the cutscene from N  Alignment (Elemental Bonds legendary quest) should now be visible before she lands on it.