Instructions for Apple.
Instructions for Android.
Some Important Information
Session Keys
A session key is a unique code that identifies you and your device. If you log out of your APP and then log in again without reloading his session key then people you have been communications with will see a warning sign near your messages indicating things have been tampered with. Before you upgrade a phone or logout of the APP, save the session key so it can be reloaded when you log back in. There are some other ways to recover that are in the detailed instructions but with out this key things become difficult.
Message Keys
Every time you send a message it is encrypted with a unique key and for each device you have. Your app keeps these keys. If you delete the APP, change phone etc and have not backed up the keys, you will not be able to see the messages you have received in the past. This is part of the security mechanism.
Account Password
If you forget your password there is recovery process if you have entered an email. Without the Session and Message keys you will likely lose messages but you can still start new messages with your connections (friends/rooms/communities). It is worth while setting up a second device such as a laptop which can help recover messages.