Log out and exit Steam
Navigate to the folder where Steam is installed (by default: C:\Program Files\Steam\)
Delete all of the files and folders except the SteamApps folder and Steam.exe
Cut and paste your Steam folder to the new location, for example: D:\Games\Steam\
Launch Steam
Steam will briefly update and then you will be ready to play
All future game content will be downloaded to the new folder D:\Games\Steam\Steamapps\