how to change the enter passcode screen from numbers to letters?

You can only do that if you turn Simple Passcode off. Is that what you're trying to do?

If your phone is disabled due to the wrong passcode being entered, do the follwing:

Either force the phone into recovery mode, as described here:

http://support.apple.com/kb/ht1808

Or, login to iCloud.com, on your computer, locate the phone, then erase it.

Either will remove the passcode.

Nothing will be deleted at iCloud.com, but all data will be erased from your phone. If no backup to restore from, then your data is gone.

You don't have a choice here, either restore your phone or don't use it.