Updated for the RC7 release
Due to several issues with the RC6 package, RC7 has now been released. If you have already updated to RC6, please proceed with the update to RC7. If you are still using RC5, you can update directly to RC7 without going to RC6.
With the substantial changes in RC6/7 comes the burden of actually having to update everything. As has been explained in detail here, these changes were necessary and lay the road for more stability in the future.
Unfortunately, this also means that all pre-RC6 styles and many MODs will no longer work. If you have been modifying your installation, then you will likely encounter conflicts if running the auto updater. To an even larger extent, you will probably not be able to "FIND" code blocks when installing new MODs.
If your board is particularly modified, it may not be possible to use the automatic package. In this case, I would suggest uploading fresh copies of all files (except config.php) and running install/database_update.php. This will remove all MODs that have been installed, but should get everything back up and working. If you decide to use this method be sure to make backups of your files beforehand.
If you are using a template other than prosilver or subsilver2, you will receive "invalid form" errors on a large number of pages. You will also be unable to reauthenticate when entering the administration panel. Both of these issues have to do with outdated code in the style you are using. 77 files have been modified between RC5 and RC6, as well as a few more in RC7. Thanks to Raimon, you can find a full list of these changes here: http://www.phpbb.com/community/viewtopi ... 4&t=585027
If you do not wish to make the changes manually, then I suggest temporarily reverting to prosilver/subsilver2 or another style that has been updated while the author of your style updates the package. If you are unable to access the administration panel to change the default style, make the changes here or use the workaround here.
If you are experiencing problems with avatars or attachments (either not showing up, or not uploading), please check the folder permissions. So far, this has been the fix for everyone reporting this problem. CHMOD the folders/files to 777 and then back to their proper setting.
/downloads/ - 755
/downloads/file.php - 644
Finally, we realize that this will not be the smoothest update for everyone, but hope that you will hold out for the better times that these changes will bring.
I'll leave this open for questions and/or fixes. This post will be updated with links to common fixes (either by me or other team members).