Editing the SSHD_CONFIG File in REHL

Hello Experts!

    I have a quick question on editing the sshd_config file in RedHat.  What I'm trying to do is add the ciphers option in the sshd_config by adding the following line:   Ciphers aes128-ctr,aes192-ctr,aes256-ctr

    Pardon my ignorance here, I'm rather new to the Linux world.   When editing the sshd_config file, I assume that I can add that line anywhere within the sshd_config file?   Once I've added that line, is there a particular command needed to save my work?  

 Basically, what I'm doing now to "attempt" this is this:   I'll log in as Root, then will do a:  # vim /etc/ssh/sshd_config it will open and ask me if I want to Open, Delete, Edit, etc the file.  I choose "E" for Edit, then I went in and added the line: Ciphers aes128-ctr,aes192-ctr,aes256-ctr at the very bottom of the config file, then will click the X to close out of the session, thinking it would save my changes.  When I try and go back into the sshd_config file, it's telling me that it's detected a SWP file and I cannot see the line:  Ciphers aes128-ctr,aes192-ctr,aes256-ctr that I added to it, it's reverting back.    Thank you in advance for your help!