To save your account, you can go to the "Data Transfer Settings" from the Gear icon when you are in-game, then choose "Data Transfer with KONAMI ID". Register a new account if you don't have it yet and bind it to the account.
June 7th update
Recently, there were problems associated with making a KONAMI ID, which has now been fixed, but a new method to save your account is now added as well. You can now set up a data transfer to back up your account using Google Play Games. Simply log in to your Google Play account when you are in-game, and choose "Data Transfer Setup (Google Play Games)" in the Data Transfer Settings.
Setting up a data transfer
1. Tap a gear-shaped icon
2. Tap "Data Transfer Settings"
3. KONAMI ID or Google Play Games
You can link your game data to both a KONAMI ID, or Google Play Games. In case one of them isn't available, you can use the other to restore your data. You can link your game data to a different KONAMI ID, or Google Play Games if you accidentally used a different or wrong account, but it will replace the previous one. If you plan on transferring from using Google Play services to iTunes, make sure you use KONAMI ID instead.
3.5 KONAMI ID
If you are using KONAMI ID, login or generate first before linking your game data to it.
If you did everything correctly, your account is now saved so you don't have to worry about losing it if your phone is broken, a data error or if you uninstall the game. You will have to re-download all of the data files though.
Using your account on a different device
When you want to use your account on a different device, you will need to log in to your account. When you first launch the game, before you tap "INITIATE LINK!", tap the button below for "Data Transfer" and log into your account.
If you set up using a KONAMI ID, simply log in to your registered account that you linked your game data to. If you are using Google Play Services, make sure you are logged in to the right account.
If you lost your account before you managed set up a data transfer, you can try recovering it from the "Contact Us" and "Submit Player Data Recovery Inquiry" from the gear icon in-game.
1. Tap a gear-shaped icon
2. Tap "Contact Us"
3. Read through the description & Tap the button
4. Submit inquiry
You will need to provide your information as accurately as possible for them to be able to identify that it is yours. Simply giving them your player ID is not enough, since anyone can look that up and is not a very reliable information. Try to remember stuff like deck names, latest card obtained of certain rarity or what device you used it on.
Next step is to wait patiently and check your email for a reply from their support team.
No, just do it one time. Everything is gonna be back-up automatically.
Yousef 3days ago
I did a mistake, instead of transfer my account and progress in the game to my new phone I transfer my new account to my old phone so my old account was deleted and I did emailed you about this, can you help my faster!?
No, you don't have to back it up again. The progress on your old phone is automatically saved.
?????? 4days ago
I used to play with my iphone 6.
I remmember my player ID was "??????".
I stopped playing by the end of the Toon world event, in which I unlocked most of the things if not everything. I believe i also had a level 30 yugi. Don't remember much else unfortunately. Please is there any way I could recover that game data?
This needs to be updated for the new transfer methods (see in game notifications 6/6/2017).
Also from my personal experience, it seems that restoring by Konami ID is not working (including fresh installs of the game). If you use to use Konami ID for account switching on the same device, don't at this time. It wants to link the ID to the current progress on the device rather than the ID you are authenticating with. Device specific backup and restore has been the only way I was able to regain access to my separate accounts. This has been noted on both Android and iOS.