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 Rehabilitation Engineering and Assistive Technology Society of North America (RESNA) has links to programs that offer training. 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 Accessible Technology website to explore this option.