Updating your theme can be achieved without using the features of the Theme Updater Pro Plan. The manual approach described in this article is the standard process for updating Shopify themes.
Obtain the latest theme version
In your Shopify admin, open the Themes page of your Online Store.
If there is a new version available for your theme, click the notification with the blue dot to reveal the version details.
Click Add to theme library then browse the changes included in the latest version by clicking View release notes.
You can also access the latest version of the theme with Theme Updater (no paid plan required).
Install default theme with Theme Updater
Transfer content to the latest version
Before publishing the theme, content must be transferred to the new theme to match the previous theme's sections, template and general theme settings, and customizations. Also, be sure to reinstall apps for the new version.
Open the theme editor for both versions of the theme in separate windows (to do this, right-click the Customize link next to each theme, then select Open in new tab).
By placing them side-by-side in separate windows, you can go section-by-section and setting-by-setting to match the configurations.
Rebuild page sections
Add the same sequence of sections to your home page and other templates, then copy the text from the previous theme to paste into the new sections.
Images in sections will need to be added but will be available in the store library (no need to upload again).
Copy template and theme setting configurations
Templates for the product pages, collection pages, etc. often have their own settings. Open these templates with the page selector, then open their section to reveal the settings. Adjust the settings for the new version to reflect the previous configuration.
Also, complete this process for the Theme settings.
To match Colors settings, for instance, copy the hex code (e.g. #3c3c3c) and paste it to the same field in the new version.
Migrate custom code
Next, you'll need to migrate any custom code added to your Liquid files. Ideally, custom code will be documented so you can easily find it. However, if this is not the case, you can use the file comparison tool Diffchecker to help you remember where you've added custom code.
Follow the link to Diffchecker. Copy and paste the liquid file that had custom code added to it into the Original Text field. Then copy and paste the corresponding file of the new version into the Changed Text field. Click Find Difference.
Any apps that add code to your theme's files will need to be reinstalled. This can be an automatic or manual process depending on the app, so confirm with the app's developer whether removing and adding the app is enough to connect with the new version.
Find the app documentation through the App Store to follow the installation instructions. If you're unfamiliar with this process, you can go to Apps and click View details to find the support email for that Shopify app.