Assuming you have setup SpamAssassin on your dedicated remote box.
You will then continue to configure your cPanel servers to connect to them.
In WHMC go to Exim Configuration Manager - Advanced Editor
Search for "spamd_address"
cPanel defaults to 127.0.0.1 783 retry=30s tmo=3m
You will now need to edit your configuration to with the IP address of your new setup. I'd also recommend to tuning rety and tmo based on the load of your remote SA server.
This option is available when Exim is compiled with the content-scanning extension. It specifies how Exim connects to SpamAssassin’s spamd daemon. The default value is