Hey Galini,
for the future, the most widely recommended way to make custom changes to themes is by using child themes – this way you can ensure that your customizations are stored separately and are not voided during an update.
Regarding the issue at hand, the code that governs the menu is located in the “header.php” file, thus you can first try updating just that one and see if it resolves the menu issue. In general, to update the theme, you need to unzip the “Kanso_Installable” archive found within the primary ThemeForest archive, and upload all contents of the resulting “Kanso” folder into the analogous folder on your server, i.e. /wp-content/themes/Kanso; in this case, you can try uploading just the header.php file. I would also highly recommend making a full back-up of your website (incl. database) every time before updating. My personal favourite plugin for that is Updraft Plus.
Let me know if you face any problems with the process!
Sincerely,
Andrii / Satori Studio