Public Licenses questions to get to know about it....

Hello everybody,,
I have a plugin installed on my server and it's a simple decent plugin that is critical for the players on my server. It has "All Rights Reserved" for the license information and hasn't been updated in awhile.

The only problem with the plugin is that there is a bad spelling error in one of the messages sent to players that makes me cringe every time.

My questions are: Is it legal for me to modify the code of this plugin? To open up the code in jd-gui and copy pasta into eclipse, leave everything the same, fix the spelling error, and then export and put it back on my server? Should I try to match the names of the packages and such? Does it really matter as long as I do not intend to redistribute it and only use it for my server? Or is it totally illegal altogether to modify someone else's code? Does the type of license matter?