Moving WordPress site to an existing WordPress site

You’re probably looking for the export/import tools. From the dashboard you can use the export tool to create a file with all the blog posts, comments and users from one blog, and then your other install use the import tool to merge them into the blog you wish to keep. You can map old users to existing users during import.

The export file serves as a kind of backup, but it will only import images whilst the old site is still online, as they aren’t embedded in the export file. So don’t delete the old site until AFTER importing.