how do i undo erase this device ?

Sorry, I cannot help with a computer that is not a Mac. I have no idea how Windows functions. On a Mac, contents are gone after you erase them. You can try a file recovery software or company, but it tends to be expensive.

What do you mean you erased the computer? Do you mean you wiped the hard drive and re-installed an operating system, or did you just delete the data from your phone? If you just erased the data you saved from your phone, look in the Recyle Bin for your data.

You dont

You cannot undo an erase - if you erased it, the content is gone. Is it your computer?

You will need to post some relevant information such as what model Mac/which OS version if you need more help.