In the wallet, near where you made the backup (under settings) there should be an option to restore a wallet from a backup.
Note: When you restore a wallet from a backup, your 'current' wallet will go away. If you have any tokens in there, be sure to back it up too before it is replaced.
This is why I deleted wallet... It ask that I am in single address wallet

When I want to change this I can't

Hmm, sorry - I don't know. You could try contacting the Byteball team. Here is their official post: https://steemit.com/steemit/@punqtured/official-byteball-airdrop-to-steemians
Ok, thanks!