how to insert new data to an already existing table?

how to insert new data to an already existing table?

in my table I have email but I also want to add more data to that table I
was thinking of adding
insert into but that adds the new data to a new record in the database but
I want to add it to the same record as email. what im doing is when
someone signs up it saves that data to a separate table and only saves
email to the table I want to add more data to. how can I link the email
address after login to add more data to the table where the email address
is ? please help