Can I transfer a USA transit visa to a visit visa online without going to the embassy in the USA?
No. There is no such thing as "transferring" a visa from one type to the other. You will have to make a new appointment, fill out the DS-160 again, pay the fee again, and go through basically the same process to get a B-1/B-2 visa. Although not required, they will probably mark your C-1 visa "cancelled without prejudice" to avoid confusion at the border. You can transit on a B-1/B-2 visa, so there's never a need to use the old C-1.
Just to be clear, if you told the consulate you wanted a visa for transit purposes, but you actually have a B-1/B-2 visa in your passport, you already have the correct visa type. There is no need to do anything.