upgrade from wp3.5 to wp3.9.1 – error on auto upgrade

Hi,

I’m trying to upgrade my blog from wp3.5 to wp3.9.1.

When I try to do so via auto-upgrade via the dashboard I receive the following error:

Fatal error: Call to undefined function wp_safe_remote_get() in /websites/123reg/LinuxPackage21/*e/*u/*s/j***************y.com/public_html/e************r/wp-admin/includes/file.php on line 497

I am currently running the default theme and have renamed the other themes to theme-old via ftp. I have also deactivated my plugins.

I believe when I tried to upgrade initially I had not deactivated all of my plugins. I also run wordpress via my host (123reg) and I have suspicions that this may be causing an issue.

I am now unable to do pretty much anything and am confronted by similar errors whatever I try to do.

Can anyone suggest a resolution please? I imagine it will be a manual upload of upgrade files via ftp however am not very confident in doing so.

Any help would be greatly appreciated – thank you.