You first have to install Root Explorer.
It allows you to change permissions for your files so you can move, delete, rename, etc. them.
While in Root Explorer, you will see a button near the top that says, "Mount R/W"
That button switches between read-only and read-write permissions for the folder you are viewing. To change a file, you need to tap that button.
If it says, "Mount R/O," then you have permission to change files.
There is a good video on YouTube that shows how to use Root Explorer at:
He actually demonstrates how to rename a file in the video.
So, in Root Explorer, navigate to /system/app folder.
Tap the "Mount R/W" button so it says "Mount R/O"
long press on calendar.apk and choose rename. Rename to calendar_old.apk.
Find the correct calendar.apk on your sd card, long press on it and choose copy.
Navigate back to /system/app folder.
tap on Paste.
Tap the "Mount R/O" button to lock your permissions.
You are done!
(Hopefully, Dexter will include the working calendar in his latest ROM)