Creating new version of migrations

I have a migration file like [datetime]_create_posts.rb .

Now I want [anotherdatetime]_create_posts.rb with some changes different
from older migration that I can run with ... rake db:migrate

How can I create [anotherdatetime]_create_posts.rb, which is basically a
new version?

you probably don't want to *create* the posts table again, you likely
want to change the existing schema. just make a new migration like

"ruby script/generate migration 'do_something_else_to_posts' "

and then use the add_column, remove_column, rename_column, etc...
methods for migrations.