Erlang : Mnesia : Updating a single field value in a row

I have an mnesia table with three fields, i, a and b, created using the record -record(rec, {i, a,b}). Now I insert a row into the table as: mnesia:transaction( fun() -> mnesia:write("T", #rec{i=1, a=2, b=3}, write) end ). Now what do I do if I want to update this row, and change only the value of a to 10, while leaving i and b with the same values? Is there any SQL equivalent like "UPDATE T SET a