If you recently decided to start using 2FAS, these are the things you can do:

If you have been using Google Authenticator and want to move to our app, you can simply transfer your codes using “Transfer accounts” in the Google Auth app, and scan exported QR code with the 2FAS app, using a QR code scanner.
If you still have an access to your old 2FA app, you can use it to log in to your accounts and then re-configure 2FA using the 2FAS app. You can scan new QR codes or add services manually to the 2FAS app and re-type your service name and the Service Key.
If you don’t have access to your old 2FA app, but you have written down or saved the backup security codes for your service’s account, then you can use one of them to log in and re-configure 2FA for your services (e.g. Facebook) with the 2FAS app.
If you don’t have access to your old 2FA app or the backup security codes, contact the service’s Support Team (e.g. Facebook Support Team). They will probably enable an alternative authentication method such as SMS, which will allow you to recover access to your account. Unfortunately, it may happen that due to the given service’s policy, it will be impossible.
If you upgrade your iPhone, by restoring a backup, you should be able to log in to your accounts as usual. Then simply re-configure 2FA using the 2FAS app.