Hibernate version: 2.1

This is not a bug or a defect, just a query. I am managing my database using hibernate and since it is still in development stages, there are frequent schema changes (additons, deletions, merges of columns, changing uniqueness properties of columns etc.) I would like some guidance as to what 's the best way to update your DB to reflect the new scheme and it's properties without losing the data? I am new to DBs and so maybe you will just refer me to a good DBA guide. Even that works.

TIA,
Gaurav