"Let say,there is one table employee with millions of record and we want to increase the column size address_col(10) to address_col(20)"

Doing this command will take *no time*, because no data needs to change.

SQL> create table t ( address varchar2(10));

Table created.

SQL> insert /*+ APPEND */ into t
  2  select rownum
  3  from ( select 1 from dual connect by level <= 1000 ),
  4  ( select 1 from dual connect by level <= 10000 )
  5  /

10000000 rows created.

SQL> set timing on
SQL> alter table t modify address varchar2(50);

Table altered.

Elapsed: 00:00:00.05

but anyway. If you need to backup a column, you can take a copy of the primary key columns, and the column needed, eg

create table backup_my_table
as select pk_col1, pk_col2, my_column
from my_table;

You need the primary key columns so you could map the data back to the original table if you needed to.