Whichever method you choose you should make sure to have no pre-installed mod or to have only mods compatible with the version of Forge you are about to install.
To remove your old mods stop the server and then delete the "mods" and "config" folders.
Method 1 (simple)
Connect to the main control panel and go to the "game mods." Choose one of the forge mod version proposed in the drop-down list and click "Install" and then wait for the confirmation email.
Method 2 (advanced)
Download the desired forge version HERE (choose "Installer" to download the forge installer).
Run the installer and choose to "Install server" to an empty folder on your PC.
Once the server is installed on your PC rename the .jar forge file (named forge-XX-XXXX-universal.jar where X are version numbers) to forgemod.jar
Connect to mcmyadmin and stop the server.
Now connect to your server via FTP.
Remove any .jar files present at the root of the server such as minecraft_server.jar, craftbukkit.jar etc.
Upload the content of your forge server installation (from your PC) to your server.
Connect to mcmyadmin, go to Control > server settings and change the "server-type" setting to "ForgeMod".
Start the server from mcmyadmin.
Forge is now installed, make sure that the server starts properly by checking its status on mcmyadmin and from the console or the log if needed.
You can now add mods (as long as they are multiplayer compatible and compatible with the version of forge you have installed) simply by uploading them to the "mods" folder of your server.
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