Unzip and copy SCOREDIT.EXE and the documentation files to your \Program Files\Ararat Software\ScorEdit 3 folder, overwriting the existing one. You may want to make a backup first. If necessary please open Global Settings and reset the library path.
Copy the files SYNTAX.TXT and CHANGELOG.TXT to the ScorEdit folder that resides in your User folder (on WinXP this is called Documents and Settings). Usually the path is something like c:\USER\<username>\LOCAL\APPLICATION DATA\ARARAT SOFTWARE\SCOREDIT3.