The place to start is with a script that’s closest to your needs in this folder
In particular you might find the Generic Database script useful. Also have a look through https://meta.discourse.org/tag/migration to see if others have done a migration similar to yours.
After you’ve reviewed some of those scripts and topics and gotten to grip with what this involves, come back here to ask us if you have specific questions or get stuck with something and we can help you through it.