First, because I've said it many times before, and you paid no attention:
BACK UP YOUR SYSTEM BEFORE YOU UPDATE IT!!!
Thank you. I hope I was sufficiently clear. Now, to your question:
Use "i" only when you have no local changes to a file. It
installs the file directly from the fileset, and if you had any local changes, they will be lost. If you make a mistake, you can recover from the back up in /var/tmp, or, failing that,
from the back up you should have taken.
Use "m" when you need to "merge" your local changes and a revised file. By default, the tool used is sdiff(1).
Please,
read that man page, and
practice merging with a few text files you create yourself, so that you understand how to use the tool.
This post may seem like a spoon feeding. It is. You are addicted to the spoon, my friend.