The difference between the two questions "how are you?" and

"how was your day?".

When someone asks, "how are you?" they want to know how you are feeling, emotionally.You would respond in the present tense.
A common response would be "I am good thanks, how are you?"

When someone asks, "how was your day?" they want to know what happened in your day and how it made you feel then. You would respond in the past tense.
A common response would be "My day was tiring as I had a lot of lessons in school today." After replying to the question, you can respond by asking them with the phrase "how was yours?"
I hope this can be helpful to some people.

Yours,
Elisha :)