Follow these steps:
Backup your current theme: Before making any changes, ensure you have a backup of your current theme. This includes all theme files, customizations, and settings. You can use plugins like UpdraftPlus or manually download the theme files via FTP.
Identify and note customizations: Check your current theme for any customizations, such as custom CSS, functions.php modifications, or template files. Make a note of these changes, as you'll need to replicate them in the new theme.
Choose a new theme: Select a new theme that meets your needs and requirements. You can choose from free or premium themes available on the WordPress Theme Directory or third-party marketplaces like ThemeForest.
Install and activate the new theme: Install the new theme on your WordPress site by uploading the theme files through the WordPress admin area or FTP. Once installed, activate the new theme.
Configure the new theme: Set up the new theme by configuring its settings, layout, and design. You may need to replicate any customizations from your old theme, such as adding custom CSS or modifying the functions.php file.
Test the new theme: Before going live, thoroughly test the new theme on your staging or development site. Check for compatibility issues with plugins, widgets, and other theme features.
Migrate content: If you're migrating from an old site to a new one, ensure you migrate all content, such as posts, pages, media, and users. You can use plugins like All-in-One WP Migration or manually export/import the content.
Update internal links: After migrating, update internal links to ensure they point to the correct pages on your new site. You can use plugins like Better Search Replace or manually update the links in your database.
Redirect old URLs: If you've changed your site's URL structure, set up redirects to ensure users and search engines can find your content. You can use plugins like Redirection or manually add redirects to your .htaccess file.
Monitor your site: After going live with the new theme, monitor your site for any issues or errors. Check for broken links, compatibility issues, and performance issues.
Make final adjustments: Based on your monitoring, make any final adjustments to your new theme. This may include fixing bugs, optimizing performance, or making design tweaks.