You are traveling upstream on a river. you see a green square daymark. what should you do

Upstream or upriver, this is the term used when you are travelling against the direction of flow of river and day marks or day markers are daytime identifiers.

When you see the green square daymark, you should pass left means keep the mark on your left port side.

Colored marks or other symbols tells us or indicates something, like white with red vertical stripes is a mark that is telling us that the water is safe on all sides.