SQL & PL/SQL

I have the table below. I want to loop through the records and update column "CUML_UNITS_TILL_POSITIVE".

I want to start at the latest "YEAR_CD". So 2013 in this dataset.

Then I will declare a variable called "X_CUML_UNITS_TILL_POSITIVE" set to be 0 initially.

I want to update "CUML_UNITS_TILL_POSITIVE" for 2013 to be the "CLOSING_UNITS" + "X_CUML_UNITS_TILL_POSITIVE"  for that year if the "X_CUML_UNITS_TILL_POSITIVE" are <0 else set equal to "CLOSING_UNITS".

Then I will set "X_CUML_UNITS_TILL_POSITIVE" = "CUML_UNITS_TILL_POSITIVE" SO THAT IT IS ALWAYS LAST "CUML_UNITS_TILL_POSITIVE"

The code I tried to use in SQL Developer is: Its doing anything. Please help if you can?

DECLARE 
  X_CUML_UNITS_TILL_POSITIVE NUMBER := 0;
 
 
BEGIN 
  FOR i IN '2013'..'1997' LOOP 
      IF YEAR_CD <= i AND X_CUML_UNITS_TILL_POSITIVE >= 0 THEN   
        UPDATE CPC_ABS_UL_YEARLY_BREAKDOWN
        SET CUML_UNITS_TILL_POSITIVE = closing_units;
      ELSE
        UPDATE CPC_ABS_UL_YEARLY_BREAKDOWN
        SET CUML_UNITS_TILL_POSITIVE = closing_units;
        END IF; 
      X_CUML_UNITS_TILL_POSITIVE := X_CUML_UNITS_TILL_POSITIVE + CUML_UNITS_TILL_POSITIVE; 
  END LOOP; 
  COMMIT; 
END;

So the table should look like....below after the update: