Typcasting

I have a field in my table that is of type varchar. I'm trying
to do a bit of tidying up and since that field contains only
number, I want to convert the field to an integer type. So I
create a new table and modify/remove the fields as necessary
and then move the relevant data over to the new table. But
when I try to insert the field that I modified from varchar to
integer, I get an error because the type of data doesn't match
up. I'm told that I need to cast the value manually.
I've looked up the CAST function and tried everything that I
could think of but nothing seems to work. I can't, for the life
of me, move the data over.
I can I cast a field value? Is there any other way to do what
I'm trying to do?

Chris