Actually, it’s a good question. At the moment the addon doesn’t offer such a function. So you would have to do that manually.
In that case, it depends on wether you used ‘offload geometry’ or not. If you did you will have to navigate to the proxy.blend and do the adjustments there. There was an addon called ‘Edit linked library’ which can open linked files in one click or so but I don’t know if it was or will be ported to 2.8.
If you didn’t, you can set the outliner to ‘Blend File’ which displays all datablocks in your file. In ‘Collections’ you will find the proxy collection with the highpoly (hi) and lowpoly (lo) object. You can link the original highpoly model to the scene and do your adjustments. Note that the vertex cloud proxy will not adap to the changes to the highpoly mesh. If you do a lot of changes I would suggest you create a new proxy to keep the proxy display accurate.
For local materials it’s a little easier. You can go to the shader editor and select the material you desire to change. For preview you can off course use the pre render button to make the object visible in cycles viewport render.