Putting output back in the same file it came from

Here is what I am trying to do.  I want to read all the files in a
directory searching for a specific string, if the string is found I want
it to be either changed or deleted depending on what I enter as a
replacement string.  I know I can do it by putting the output to a
separate file and then just copying the new file over the old one but is
there a way I can just change/delete that string in the original file?
 
Thanks,
 
Dylan

• Putting output back in the same file it came from by Dylan Boudreau
• RE: Putting output back in the same file it came from by Bob Showalter
• Re: Putting output back in the same file it came from by John W. Krahn