Many universities offer training in assistive technology and the design of accessible technology as individual courses and/or as part of certification programs. Whether you want to pursue a degree or certificate or just take one course, be clear about the outcomes you desire and the methods of instruction you prefer. Some courses focus on book learning; others include active learning, with adequate time to master new skills. Many programs have at least an aspect of online education; some are offered only as distance learning programs.
The TRACE Center  has links to programs that offer training. Some of the links lead to training programs that are offered free of charge. EASI  also offers low-cost online courses on assistive technology and the design of accessible technology.
The University of Washington's DO-IT (Disabilities, Opportunities, Internetworking, and Technology) Center offers free web-based tutorials, streaming video training, and links to useful resources for those who prefer self-directed learning. Consult DO-IT's Technology and Universal Design  website to explore this option.
-  TRACE Center
-  EASI
-  Technology and Universal Design