At the age of 26, your daughter needs to make some decisions of her own.  Does she want to go back to school? Does she want to improve your reading, math, and social skills or do you want her to?  Depending on her answers, you can guide her to get the help she needs and wants.  

I would find out want her interests are and work form there.  Does she have a career in mind?  This may take several conversations with her to determine what she wants to do with her life.  You may hear somethings you aren't crazy about, but guide her.  Point out the pros and the cons if needed.  

Often public libraries have literacy programs for adults.  As for social skills, the best way is to practice in real life situations.  For example, when traveling with your daughter, have her be the one to ask for directions, check in, or even order for you while dining out.  Need to call to make an appointment, she can do it.  Make it fun!

Hope that helps.

One thing you may look into is having her take some fundamental classes at your local college. She can get some support from the Disability Services program. This will allow her to slowly start back into the education world and to improve her basic skills. She will not need to  take a full load of classes only a few or just one.