Actually, you just upload the new files right over the old ones. When you then visit your admin page, it should say that it's discovered an old version of Serendipity and it's upgrading for you. It'll make any database changes that are necessary and report that it's upgraded.
you just upload the new files right over the old ones
upload full distrib of serenditipy in root directory blog?
Pretty much. Make sure your directory names match: if you installed Serendipity in serendipity/ like the default, then upload all the files to serendipity/, overwriting the existing ones. If you changed the name to dynamic/blog/, upload all the files to dynamic/blog/ instead (again, overwriting the existing ones).
Then visit your admin page and Serendipity will automatically upgrade.