Server icon is an image in PNG format, which must be within the 64x64 pixel size limit. You would need to name it server-icon.png and upload to your server:

Once that's done you will see this icon updated:

MOTD can be very easily updated at any time. All you need to do - log into the Game Panel, manage your instance and open Configuration → Minecraft menu. There you will see the option to update MOTD:

Once you change it, this text will get updated:

That's it! Now you know how to change your server icon and MOTD 😊