So am not a Garmin user (Apple) but for years now I will regularly wash my band with soap and water and regularly wipe the back of the face (the sensor part that comes in contact with your skin) with a cloth soaked lightly in alcohol.

If I am doing some outdoor work or a lot of exercise then I will always do this as well afterwards. My wife ran into some issues with her wrists after getting hers where she developed a rash and found that regularly cleaning of the watch - both band and the sensor area stopped this from happening.