Always keep backup of your database, uploads(images) etc. before WordPress / Theme update.

• Always keep backup.
  Theme update do not erases data,
  means it do not delete any of your pages / posts / content / settings of any plugins / customizer etc.  
   

  Still while using WordPress it is always advised to keep backup if under unforeseen circumstances database gets erased or corrupted. There are many reasons why this could happen.
  Please check CODEX article at WordPress Codex: WordPress Backups

   

  Always keep backup of following:

  – Database
  – Uploads (images)
  – Custom CSS (if you have any)
  – Child Theme (if used)

   
• Clear Cache – Browser Cache / Server Cache / Plugin Cache
  Make sure to do this.   

Please make sure you have not changed theme files. Use child theme if you need to change theme files. No support will be provided for update queries if you have changed main theme files itself. Refer Child Theme for more details.