how to update all rows of a column in oracle with a start value of 500 and incrementing in oracle

I won't use PL/SQL since it could be done in plain SQL.

You could use a SEQUENCE starting with 500 and incremented by 1.

For example,

set up

SQL> DROP SEQUENCE s ;

Sequence dropped.

SQL>
SQL> CREATE SEQUENCE s START WITH 500 INCREMENT BY 1;

Sequence created.

SQL>
SQL> DROP TABLE t PURGE;

Table dropped.

SQL>
SQL> CREATE TABLE t AS SELECT LEVEL id FROM dual CONNECT BY LEVEL < =20;

Table created.

SQL>
SQL> SELECT * FROM t;

        ID
----------
         1
         2
         3
         4
         5
         6
         7
         8
         9
        10
        11

        ID
----------
        12
        13
        14
        15
        16
        17
        18
        19
        20

20 rows selected.

SQL>

Now, let's update the table with the sequence.

SQL> UPDATE t SET ID = s.nextval;

20 rows updated.

SQL>
SQL> SELECT * FROM t;

        ID
----------
       500
       501
       502
       503
       504
       505
       506
       507
       508
       509
       510

        ID
----------
       511
       512
       513
       514
       515
       516
       517
       518
       519

20 rows selected.

SQL>

So, you have all the rows updated with the sequence starting with 500 and incremented by 1.

Please try like this,

DECLARE
    VAL = 500;
BEGIN
    FOR REC IN ( SELECT
                   *
              FROM
                    Table1
             )
    LOOP


        UPDATE Table1  SET  col1   = VAL WHERE  COL1 = REC.COL1 ;
        VAL = VAL +1;
    END LOOP;
END;