1. Make a copy of your emule config folder. Do that from time to time in any case, even if you are not changing clients. That folder contains all your settings and credits, so backup.
2. Download eMule Plus (binary (zip) or installer (exe)). Extract or install, then, before starting the client, copy the files from your old config folder into the new config folder.
3. Your firewall (if it is any good) will detect that this is a different emule.exe, so it will ask you again. Give it permission to connect to and accept connections from the internet.
If you have a decent connection, you can even run both clients side by side. Don't use the files from the old config folder in that case - let the two clients be different - different ports, different userhash, different credits file, etc. Running more than one client with the same userhash (identity) is a good way of getting banned by servers and clients. Also, using the same ports for both clients will result in confusion and LowID, so keep them different.
If you have done port forwarding for eMule, do the same for eMUle Plus also. If you copied the files from the old config folder and are running only one client, eMule Plus will use the same ports you were using earlier, so you don't need to do anything. If you want to run both clients side by side, you need to make another set of port forwarding rules, this time for the ports that eMule Plus is using.
If you have any problems or questions, go over to
http://emuleplus.info/forum/ and ask there. Good idea to read the FAQ, especially 'Main Interface Explained' and 'Preferences sections'.