Follow the simple steps to recover from the above wordpress upgrade error
step 1 .Create a folder, which may be missing:
/wp-content/mu-plugins
step 2. Create a php file inside that folder and name upgrade_fix.php
put the following code into the php file and save
add_filter('https_ssl_verify', '__return_false');
add_filter('https_local_ssl_verify', '__return_false');
step3. Go directly and run the network upgrade again, the issue should be solved.