I need to fix my Employer's state ID number on my W-2. But I am unable to actually change the existing numbers. How do I fix this?

It depends. You should be able to change the number without any problem so you may want to delete the W-2 and then re-enter it.  After you delete it and before you re-enter it you should clear your cache.  It handles many issues that seem nonsensical on a regular basis. 

• Close your TurboTax return first.

Watch to be sure you are selecting 'all time' as example.  Do not use selections like 'last hour' for those browsers that give  you options.

• How to clear your cache

Once this is complete open your return and select to enter your income. 

1. Select Federal in the left menu and then Wages & Income, OR 
2. Search (upper right) type w2 > Jump to... link
3. Begin your entry

You can use the federal EIN for the state number if necessary as well.