I want to change my dependent and let my son dad claim instead but it won't let me change it

You can delete your dependent by following the steps below. Let me know what is specifically happening, if you have any difficulties following. 

1. Login and click 'Take me to my return' 
2. Next, select 'Personal Info' tab 
3. Since you've already been through this section of the software, you should be a the Personal Info Summary screen. Scroll down to your dependents name and click the blue hyperlink with the trash can icon that says Delete. 

Note: if you've already filed your return, you will not be able to delete a dependent. You'll need to amend your tax return.