I have an associated metadata.yml file. However, I see no way to incorporate it into data that is already demultiplexed. If I open the .qza file with an editor [TextWarngler], I see a metadata.yaml as follows:
Specifically, the link to the metadata guide at the bottom. In general though I would recommend you take a step back and review the documentation and tutorials we have put together at docs.qiime2.org — the fundamental ideas behind QIIME 2 are described in great detail there, and will hopefully steer you in the right direction, generally speaking. As always, we are here to help!
(Matthew Ryan Dillon)