Need to change some entries in column to NULL

I have just started updating a website that I setup with an earlier version of WA extensions. Changing it over to mysqli at the same time.

I found a few things in the actual db that I need to change. I have a phone2 field that for some has 3 different values in it. NULL which is NOW the default (it was   for some reason. Did it 7 or 8 years ago so dont know why. The other values are   and then a bunch with actual phone numbers.

I have searched online and have succeeded on messing up the table ( dropped and restored already) for a way to change the   to NULL. I dont want to change the ones with actual phone numbers. I managed to set it ALL to NULL which is when I restored a backup.

Here is one I tried that obviously didnt work..
UPDATE alumni SET phone2 = NULL WHERE phone2 = ' ';

Hoping I can get a little guidance. I am beginning to think it isn't possible. can this be done? There are 96 rows to do. I will do manually if I have to but would prefer not to..
Thanks.