If you previously purchased it on Google Play, make sure to use the same email address you made the purchase with on your device accounts settings (Android Settings > Accounts > Google) and also for your Google Play app (GooglePlay > Accounts) (delete all other accounts on the device). Then delete PicPlayPost, restart your device with the account you had made the original purchase with, and re-install PicPlayPost. Google should automatically apply the upgrade to your device when you do this.
If you purchased it on iOS/iTunes see the section below covering the topic.