Hi je,
If there is no any custom code in parent theme then no worry about data loss, plugins, menus,widgets while updating to new version. Simply delete old version and add a new version. Everything will work fine.
If there is any code customization then all customized code will be lost. To know whether there is child theme or not, there will be activated next theme activated instead of parent theme.
If you are not known whether there is any code customization or not or your developer has made any code customization then backup is very much important. login to ftp and inside public_html-> wp-content-> themes (interface)(backup/copy this interface folder to your pc).
Thank you
.
.