With Shopify 2.0 versions, we sometimes release new, updated versions of the themes that we’ve published on the Themeforest. These updated versions usually include bug fixes and new features.
1. Download the latest version
If the theme offers an updated version, then you can add the latest theme version to your store by downloading a new theme from Themeforest.
2. Upload the new theme into your store.
Navigate to Online Store> Themes> Add Theme > Upload theme.
3. Migrate your current theme manually
3.1. Move the theme configuration
Step 1: Move settings_data.json file.
On old theme Go to Theme actions > Edit code 
Open the settings_data.json file > Copy all code. 
Open the corresponding file from the new version and paste all contents then click Save button on the right-hand side to save the changes. 
Step 2: Move index.json file.
You can do the same as you did with settings_data.json file.
Step 3: In addition to copying the settings_data.json file you also need to copy all other .json files in the templates folder to new version. You can do the same as you did with settings_data.json file. 
3.2. Preserve your translation
If you are using a different language like French or simple you have to change some English words from default to your so you have to move them from the old theme version to a new theme version. This step-by-step guide walks you through how to migrate your translations manually:
Step 1: Open the Locales folder from old theme. For example, if your store uses French so then open the fr.json file. 
Step 2: Open this file from the new theme and paste all content then click Save, it will apply your old translations.
4. Move theme translations from Kalles 2.x to Kalles 3.x
In case if you want to move theme translations from Kalles 2.x to Kalles 3.x please follow this below instructions:
Step 1: Open this url on your browser: https://kalles-demo.myshopify.com/pages/compare/?preview_theme_id=
Step 2: Open the language file of Kalles 3.x and copy all content into “Json New” option of the above link.
Step 3: Open the language file of Kalles 2.x and copy all content into “Json Old” option of the above link.
Step 4: Then click on “GET NEW JSON” button to generate new json file.
Step 5: Click on “COPY JSON FILE” button to copy content of new json file and paste it back to the language file of Kalles 3.x then click Save button to update file.
5. After you add an updated theme
After you have added an updated version of your theme to your online store, you can do the following: You can work on your theme in the theme library before you publish it. Adding an Online Store 2.0 theme to your theme library doesn’t impact your current theme or any other themes in the library. When you make changes to your theme or switch to a new theme, it doesn’t affect the other parts of your account. You can play around with different theme styles and settings without worrying about the rest of the content in your store.