How to quickly increase column size in Oracle?
Your two options are (depending on the use case):

https://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:8753545300346585555

https://asktom.oracle.com/pls/asktom/asktom.search?tag=best-way-to-change-the-datatype-of-a-column-in-a-table-with-20-million