When updating to a new version, you can copy current settings from one version of the theme to another. Sometimes, a page template is not transferred successfully causing a 404 error on the Home, Collection, or Product pages.
To resolve the issue, you can manually copy the affected page(s).
How to Manually Copy Pages
In Shopify Admin, click on Online Store > Themes.
Click on the three-dot button of the desired theme.
Click on Edit Code to open the code editor.
On the Templates folder, click Add a new template.
Select the template to create as a JSON format and save it (Please note that the file name will not appear in these cases).
Each page has a different template name, therefore select the one where the 404 error appears.
Home page - index
Collection page - collection
Product page - product
You can copy the file from your older version and paste it into the new theme or you can reset your page manually from the theme editor.