To "insert" a field, you use the "update" command, since the record is already in the database.
eg.
update NamesTable set FirstName = 'H' where UserID = '12'
If you don't already have a record in the database (in the above example, the record can be identified from its field UserID), you would need to insert the record
eg.
insert into NamesTable
(UserID, FirstName)
values
('12', 'H')