You can change those settings in the Galaxy settings panel.
To open the settings panel, click on the GOG.com icon in the top left corner of the Galaxy window, and select SETTINGS from the drop-down menu.
The folder settings can be found at the bottom part of the settings window.
- Install games to - refers to the directory in which games will be installed.
- Temporary game download folder - is the folder which game files are temporarily downloaded to before the game is installed.
- Download backups & goodies to - points to the directory to which all game goodies and standalone installers are downloaded.