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.   
   
• Prefer child theme.
  • Never edit main theme’s files.
  • Also, never add your custom CSS in theme’s CSS files.
  • This way you will lose the changes made, while updating theme.
  • If you have modifications which are not possible without editing theme file, use child theme.
  • If need to add just custom CSS, you could skip using child theme.
    Use additional CSS tab of customizer, to add custom CSS.
   

  What is Child Theme?

  On Activating Child Theme Why I Lost my Customizer Settings?

   

  No support will be provided for update queries if you have changed main theme files itself.

   
• Do not rename theme folder
  Do not rename theme folder if you are not sure how to modify child theme to suit this.   
   
• Do not keep copies of theme folder in ‘wp-content > themes’.