Just to be safe, we recommend saving a backup of your vocabulary to Dropbox, Google Drive, or your computer first. You can find step-by-step instructions in the Protect, Recover, and Share Vocabulary support section.
Once you have saved a backup, make sure your device’s iOS is fully updated. For instructions, see http://support.apple.com/en-us/HT204204
Then, open the App Store app. Go to the Updates section. Scroll down to and find Proloquo2Go in the list and tap Update next to it.
You can find more information in the section Protect, recover, and share vocabulary.