`Call to undefined function bp_displayed_user_id() in C:xampphtdocsbpwp-contentpluginsbuddypressbp-corebp-core-template.php on line 1117`
I got that error too when I clicked on the update wizard. Good thing it didn’t let me run the database update yet so it was as easy as just replacing the plugin folder with the one I backed up beforehand. I tried renaming the theme folder before reverting to the old version, but it wouldn’t read the default Twenty Eleven theme.