A common, standard SQL way of doing this is:
UPDATE dest_table t1
SET
field1 = (select field1 from src_table t2 where t2.pk = t1.pk),
field2 = (select field2 from src_table t2 where t2.pk = t1.pk),
...etc.
WHERE EXISTS (select 1 from src_table t2 where t2.pk = t1.pk)
If you are using Firebird 2, you can use EXECUTE BLOCK to write a more effective statement:
EXECUTE BLOCK
AS
DECLARE VARIABLE field1 type;
DECLARE VARIABLE field2 type;
...etc.
DECLARE VARIABLE pk type;
BEGIN
for select pk, field1, field2, ... from src_table
into :pk, :field1, :field2, ...
do update dest_table set field1 = :field1, field2 = :field2, ...
where pk = :pk;
END
If you are using Firebird 1.x, you could write a stored procedure to do the job.
Beside direct SQL you can use some tool like FBExport that allows you to have better error handling (it's easier to detect the exact row and column that fails to update). Here's FBExport example, first export data to a file:
fbexport -S -Q "select pk, field1, field2, ... from src_table" -F test.fbx
Then, import into the other table:
fbexport -If -Q "update dest_table set field1 = :2, field2 = :3, ... where pk = :1" -F test.fbx
FBExport is an open source tool. The download for latest version is here:
http://www.firebirdfaq.org/fbexport.php