Exporting oracle tables on command line as sql files

As far as I know, Import/Export and Data Pump are command-line-only types of tools. But if you really wanted to use that functionality from within SQL Developer (or SQL Plus), Data Pump uses the built-in DBMS_DATAPUMP and DBMS_METADATA packages. You should be able to call those packages directly, so I would start there. There's documentation available in the Oracle® Database PL/SQL Packages and Types Reference on both of them (DBMS_METADATA and DBMS_DATAPUMP).

Not using a built in command, but...

If an UPDATE really can satisfy your requirements as your title implies, then no inserts should be necessary and the primary key must be in sync between the databases.

Your requirements don't allow remote updates, but if you can remotely query the database, then you can join the local table to the remote table to create the statements that will need to be run the remote system. Here is a demonstration:

drop table t1;
drop table t2;
create table t1 as (select level c1, level c2 from dual connect by level <=7);
create table t2 as (select * from t1);
update t2 set c2 = null where c1 in (3,6);
select * from t1;
select * from t2;

select 'UPDATE t2 SET c2 = ' || t1.c2 || ' WHERE c1=' || t1.c1 || ';' sql from t1 
join t2 on t1.c1 = t2.c1
where t1.c2 <> t2.c2 OR t2.c2 is null;

If you cannot query the remote database, you can update every row like this:

drop table t1;
create table t1 as (select level c1, level c2 from dual connect by level <=7);

select 'UPDATE t2 SET c2 = ' || t1.c2 || ' WHERE c1=' || t1.c1 || ';' sql from t1;