In order to update the theme manually, follow the next steps:
Firstly, make sure, that you updated your WordPress version is no lower than 5.5.
Before, the only way to update was to use FTP client to upload the new version. WordPress 5.5 has fixed this problem and now you can simply go to the Plugins > Add New page and upload the new version like you would upload any new plugin zip file.
WordPress will automatically detect that you already have this plugin installed and the new file contains a different version. It will then allow you to ‘Replace current with uploaded’ version.
You can do the same thing for themes as well. Simply go to the Appearance > Themes page and then click on the ‘Add New’ button at the top.
After that click on the Upload Theme button to upload your theme zip file. WordPress will automatically detect the existing theme and will give you the option to ‘Replace current with uploaded’ version.
Look at the video tutorial!
That's all!