Change post time format

I really want to change the post date format in the Default theme to the post date format that is used in
the Primrose theme.

I also want to have the post date under the post (instead of above) like Primrose.

Can anyone on here help me with that?

I do not know HTML or CSS so ideally it would be awesome if someone could just share a code snippet for me to add and/or share detailed steps how to do it.