There is no reason to loss any data during Theme Update. WordPress Posts, Pages, Contents, Menus, WidGets are stored in a MySQL Database, not in any Theme Files. So if you Remove/ Overwrite/ Replace the Theme, your data will remain as it is. If you loss your data, that may be another reason. Did you follow the Link we provided for Theme Update ? https://d5creation.com/themeupdate
Theme update is nothing but Removing and Reinstalling the Theme or Overwriting the Theme Files. No WordPress Dynamic Data is stored in Theme Files. So, there is no reason to loss any WordPress data or settings.