How do I backup and restore VMs and CTs?

Some background information:
I have 1 server connecting 4 Proxmox Nodes. They were all on the 5.0 version, but I wanted to upgrade them to 6.1. I upgraded 1 node to 6.1 and I want to backup VMs from the other 3 nodes on the 5.0 version into the 6.0 version. I also do not have any internet access. I read that I have to do it externally, but I am having trouble on that.

Thanks in advance!