Please find out steps below:

– You need back up your site (database+code), if back up make error you can use back up file.

– Copy your custom code which you change in your code.

– Download new version theme, uninstall old theme and install new theme.

– Deacitve and remove plugins which request to install in your site (this is our plugins, not plugins which you install after).

– click Begin installing plugins: http://prntscr.com/aak9j5 and install plugins again (this is new version which we changed code and you need update them).

– Paste your custom code in new theme.

– Check your site again to sure it works.