How do I change DR1778 preference? I checked that I want to mail it, but would actually prefer to send it electronically

It can't be e-filed through TurboTax, so you don't need to change anything.  But you can still e-file the forms directly through the Colorado Revenue Online system.   See link below.  

 

Use the e-FiIer Attachment option at Colorado_gov/RevenueOnline, which eliminates the requirement to mail this form.