Are you trying to change a data type of a database column?

Seems that the column is not an empty one. So before you change it, first you have to set the column as empty by updating that database column. (make sure to take a backup of the particular table) Then do your changes and finally update the column again using that backup table. 
In Oracle documentation, it suggested below way for similar scenarios

Suggested way:

Suspend constraints
Create another column with a new data type
Copy from old column to the new column
Delete old column
Rename new column to the old one
Rebuild indexes
Activate constraints.
 

If you can specify more about your rdf change, then we can help further if needed.
Normally these types of scenarios come while in upgrading projects.

Hi Pradeep,

Thank you resolved that issue ,i got one more issue.

ORA-06550: line 34, column 120:PLS-00103: Encountered the symbol "OPERATION" when expecting one of the following:   ( error at line no :141

Hi @Ramesh Talluri 
Seems there is  a syntax error in your code.
please check this 

Oracle / PLSQL: ORA-06550 Error Message (techonthenet.com)
Its difficult to guide further without that erroneous code block.