The first step is too look around the forums for an answer, their is a magical tool call search that will help you. This has been discussed many times.
Method 1: (no coding knowledge is required) Find the plugin on GitHub or find another way to contact the author Contact him/her to update the plugin Wait ... Profit Method 2: (a bit of php knowledge is most likely required) Change api in plugin.yml to 3.0.0-ALPHA12. Try loading the plugin and look out for errors Fix errors ... Profit
If you get errors, or the server will make a crash dump or it will just show the error directly in the console , it's pretty easy to fix basic plugins but when it's some big plugins and there is a lot of error (and you don't know php) just try to contact the author or create a thread on the forum on : https://forums.pmmp.io/forums/development/ for some help from the other dev on the forum !