In your WordPress Admin Panel, go to Appearance > Themes
Click on the Add New and Upload Theme buttons
Click on the Choose File button and navigate to find the Rekko.zip file on your computer and click on the Install Now button
Wait while the theme is uploaded and installed
Activate the newly installed theme
Some hostings have a restriction on a size of a file that you are uploading. If you have any warning, you will contact your hosting provider. They should increase your maximum upload size. Or you can refer this article and try doing it by yourself.
If you get the error message: "The package could not be installed. The theme is missing the style.css stylesheet.", it means that you didn't upload the theme package. You will need to extract your package and install the Rekko.zip file inside.