Microsoft has been making great strides in improving PowerApps functionality over the past year. In late 2017, the product team announced the ability to set values for Managed Metadata (aka Taxonomy) columns in PowerApps and Flow. This is great if you are using managed metadata fields in a SharePoint List, but does not work for SharePoint Document Libraries according to Connect to SharePoint from PowerApps.
So how do you update the Document Library managed metadata? Consider the fact that wherever you use the same managed metadata, the same term set, terms, and associated globally unique identifiers (GUIDs) are used.
The work-around is to access the managed metadata and combine it with the Document Library data. One way to achieve this is to create a SharePoint list that can be used for the sole purpose of hosting these metadata fields. Then, when building the PowerApp, you can pull in all the data you need from the Document Library and the metadata-hosting list and pass it to SharePoint, a Flow, or any other downstream work action to update the Document Library managed metadata.