Upgrade.txt gives procedure for upgrading your drupal installation, but its missing some essentials.
If you follow the instructions word for word on a site with contributed themes and/or modules it will certainly break.

excerpt from upgrade.txt "
6. Remove all old files and directories from the Drupal installation directory.
7. Unpack the new files and directories into the Drupal installation directory.
"
Step 12 (or step 8) should tell you to Copy your backed up contributed or custom "modules" directories to the Drupal installation directory.
Step 13 (or step 8) should tell you to Copy your backed up contributed or custom "themes" directories to the Drupal installation directory.

Comments

gábor hojtsy’s picture

gábor hojtsy
he/him
Primary language Hungarian
Location Hungary
commented
Status: Active » Closed (duplicate)

Current steps 6, 7 and 8 say this, so they already solve the issues you had problems with (by suggesting to copy the sites folder):

6.  Remove all old files and directories from the Drupal installation directory.

7.  Unpack the new files and directories into the Drupal installation directory.

8.  Copy your backed up "files" and "sites" directories to the Drupal
    installation directory. If other system files such as .htaccess or
    robots.txt were customized, re-create the modifications in the new
    versions of the files using the backups taken in step #1.