Terminal is a program in your Applications --> Utilities Folder
Double click to open it
Then enter the commands exactly as printed out above (cut & paste is great).
For a short Primer on Terminal, go to http://guides.macrumors.com/Terminal
It is a great little program and many Mac Tips are on line which set out commands that help out your Mac in many ways...worth a little time invested.
You should have a backup system for your whole HD contents - especially your boot drive. Again, lots of reference materials & tips online..use Google to find them... its sort of fun to set up a good & reliable system. I use Time Machine along with SuperDuper & an external HD.
As for saving a specific file/folder, just move it to Trash but do not empty the Trash until you are sure all is well once you have completed your changes.
Restart after you have finished and, if all is OK, you can then empty the Trash.
Hope this helps.
And I continue to hope Apple smartens up its Mail application!