Set values to NULL (and set value to string "NULL")

[p-behr]

When using UPDATE: you can set a column value to NULL by typing NULL in the field.
If the column is less than 4 characters I am not sure how to set it to null ("NULL" won't fit).
I have not had a need, but I also don't see any way to set the value to the string "NULL" (i.e. UPDATE file SET field = 'NULL').

create table qtemp.testfile
( fld1 char(4), fld2 char(3) );

insert into qtemp.testfile
values('', '');

update: select * from qtemp.testfile;

If you double-click in fld1 and type NULL it will set the value to null.
If you double-click in fld2 and attempt to type NULL it won't fit and can't set the field to null value.