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.
There are instructions in my post for taking a backup. Did you do those steps?
Yes, but don't know how to access that file. It is on my PC.
Yes. Wherever you saved it.
I know where I saved it, but don't know what to do with it? How to access it, with what program?
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