the command "change announcements" will show you the list of announcements with their extensions.
the command "disp feat" will show you the Feature Access Codes for your switch. Make note of the Announcement code.
the command "disp cos" will show you the Class of Service settings for your switch. Find a COS number that has "Console Permissions" turned to "Y".
Find the station that you want to record messages for, and then type the command "change station xxxx" and change the COS field to the number that has console permissions.
Then, go to the phone, type the Announcement Access code from step 2. When you hear dialtone, type the announcement that you want to change.
Once you hear dialtone, press 3 to delete the current message.
Wait about 15 - 30 seconds. Pick up the phone again, dial the Announcement Access Code, dial the announcement extension, then press 1 to record. To finish the recording, you can press # or simply hang up.
You can listen to the announcement by dialing the extension number.