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.