Regarding command Uses

[Gourav Khira]

Hi Team,

Actually, i want to replace any word with another.i used "sed" command but
it's not getting properly result.

*For Examples*:

THIS IS 1ST UPPER CASE LINE IN THE FILE.
 this is 1st lower case line in the file.

 THis is THe the mixed up line.

  Ram , Shyam, john, and Mohan are good students.

  But somya is not good girl

  This .........



If,I want to replace here "this" with "that" using "sed" command.


sed this that | filename


Please suggest me which command is right to use here and revert me ASAP.


Thanks & regards,

Gourav Khira