1. Download the main RSGallery2 component

On github look for the newest version (v4.2.0++) Releases of component

Inside the assets you will find a zip file with a name like Download and install the newest zip file matching this name and a newer version number

2. Install RSGallery2 component

RSGallery2 is installed like all other Joomla! extensions. Log in to the Administrator section, click on Extensions > Manage ->Install

Extension install  component view

(1) Select "upload package file" tab

(2) Drag and drop the downloaded zip file into the drop area

On successful upload you will get a message similar to following image

RSGalllery2 component updated

On upload error try to transfer (?the content of?) the zip file to the server and use "Install from folder". (Some php.ini settings restrict the size of uploaded files)

Clicking on (1) will lead to the RSGallery2 control panel