I was upgrading several forums using phpbb3 by leaving the config.php, files and images directory in place and copying the latest version over it (read their guide here).

Then I upgraded the database by running install/database_upgrade.php on your new installation. However, after this, your database may still have problems, if you receive the error:

SQL ERROR [ mysql4 ]

Field 'topic_last_post_subject' doesn't have a default value [1364]
check here.

If images in the files folder look garbled, check the file transfer mode when you are transferring files. Set it to binary, don't trust "auto" modes. If the transfer is by mistake done on ascii, images will look garbled.

Summarizing the ugrade process:

1. get the copy and unpack the copy to your disk to say, directory "trash".
2. rename the "install" subdirectory "install-old".
3. make a new directory, example "forum1".
4. copy your forum config.php file , "images",and "files" directories to the "forum1" directory.
5. copy the contents of the "trash" directory over the "forum1" directory.
6. upload the "forum1" directory using ftp with transfer type set to binary to the same level of your server, i. e. if your forum is n /public_html/forum/, your new copy will be at /public_html/forum1/
7. Run the database_update.php script on your install-old directory
8. test the forum accessing your new directory.
9. if you have the 1364 error, check the instructions above
10. if everything is ok, you may rename your old directory forum-old or delete it and rename forum1 as forum (or the name you have).