Use the mv* (move) command to rename a file. The mv command has exactly the same syntax as the cp* command. Specify the name of the file you want to rename followed by the new filename. It is always a good practice to make sure that the new name is unique; an existing file may be overwritten by the mv operation.
The file chap1 is "moved" to a new file called intro. Then the file chap2 is renamed as chap1.