1. Log into your multicraft account: https://cp.sparkedhost.us
2. Stop the server and set the JAR File to Default, be sure to save. (Picture: http://prntscr.com/kmj247)
3. Start and stop the server to allow Multicraft to generate the files.
4. Go to Files > FTP File Access, Log in with your multicraft password.
5. Click on Server_Default folder
6. Click Upload > Choose file, select your custom jar from your computer and click Submit.
If the upload fails, you will need to upload with an ftp program like Filezilla.
Check this link for information on accessing your files using Filezilla: https://billing.sparkedhost.us/knowledgebase/27/How-to-use-FileZilla.html
7. Type your custom jar name below "Default". Picture: http://prntscr.com/kmj2v4
8. Press Save
9. Start your server