How to compare string with objective-c?

How to compare two string while some object are the same? Let say I have a string ABCAAAA.

For some reason, only ONE character of the string ABCAAAA can only be changed each time. For example, I can change this string to DBCAAAA for this time.

Now the problem is : How can I ensure ONLY ONE character is changed for each time? Is there an instance method for NSString to compare two strings?

Purpose: I put each string into each UITextField to determine whether this one is editable if others had changed. I need to ensure only one is edited for each time. So if one had been edited, I will set UITextField 'senable to NO to disable editing.