Brian706 gives good advice.
I'll add this little part, don't delete the old number too fast ... instead, change the name of the number to something like "old #" or "deleted #" for a while. This way if you're in the habit of typing the number so it will automatically populate their info into the field, her info will come up, but that number will say "old" or "deleted" and you'll know to switch to the new number.
This comes from an old man who has many contacts over many decades ... my memory banks are old, tired and have been corrupted by years of alcohol enjoyment ... so my reference points are usually their first numbers or email addresses ... doing it this way helps me to quickly find the person I'm looking for.
Hope that helps.