Post pinning is a great feature, allowing you to keep a post stuck to the top of your profile, Google+ page or Community. Today, it is possible to pin posts on your collections too, but with one caveat - the feature is only live on Android currently.
If you are using Android, it is easy as tapping on any post in a collection, hitting the three-dot menu and selecting "Pin to collection". If you're not seeing the option there, you should see it within the next few days as the feature is rolled out.
We're certain that pinned posts for Google+ Collections will hit the web and iOS app, but we can't say how long they'll reach feature parity in this area.
Google+ 6.4 for Android doesn't seem to have any obvious user facing changes, but there are probably tune-ups behind the scenes such as performance improvements and stability fixes; all speculation on my part though.
Download
You can grab the latest Google+ APK on +APK Mirror by clicking the link below:
Source: Google+ on Google Play