Receive plugin updates and release notes via RSS feed here. Anytime a plugin is updated that is also contained in a Combo version it is assumed the Combo plugin also received an update even if it is not explicitly listed.
When a release note tells you to "delete old files before importing" you should do so to avoid errors. To delete the old files simply delete the Editor/PLUGIN_NAME and Plugins/PLUGIN_NAME folders
Google Jar Resolver can be a bit buggy when updating to a new version of the Play SDK (luckily this only happens once per year). We have found the best way to keep it happy is to do the following: import the plugin update, delete the xml file that Jar Resolver creates in the ProjectSettings folder, delete the Plugins/Android/play* jar and aars, restart Unity and finally run Jar Resolver manually (Assets -> Google Play Services -> Resolver Client Jars).
If you are using Unity 5.x and you get an error when doing a build referencing UnityEngine.ImageConversion (which doesn't exist in Unity 5.x) run the updater in the Tools - prime menu to fetch a version of the DLL that didn't get mangled by Unity's API updater.