You can change those settings in Galaxy settings panel.
To open settings panel, click on the GOG.com icon in top-left corner of 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, where the games will be installed to.
- Temporary game download folder - is the folder, where game files are temporarily downloaded to, before the game is installed.
- Download backups & goodies to - points to the directory, where all game goodies and standalone installers are downloaded to.