How would the solstices and equinoxes change if the earth's orbit were circular instead of elliptical. explain how a circular orbit would effect seasonal changes?

Seasons, solstices and equinoxes are related to the angular tilt of the Earth's axis of rotation as compared to the ecliptic, which is the plane of the Earth's orbit. They have nothing to do with the elliptical orbit of the Earth.

The Earth's orbit is an ellipse, but it is very nearly circular; the eccentricity is only about 3%. If the Earth's orbit were perfectly circular, we probably wouldn't notice any difference.

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The Solstice or equinox would not change. These are related to the tilt of earth.

Earth is at its closest to our star during December and January, at about 91 million miles, and farthest during June and July at 93 Million miles.

While a perfect circular orbit would not effect your questions it would also be very unlikely that it would ever occur.

As a thought experiment, If there were only Earth and our star, as each orbited while Earth is small in comparison would still pull at the star a small amount.

The Star and earth would be orbiting each other.

Given that objects have mass,

Given mass is attracted to mass,

I do not know of a method other than computer modeling,

to map the orbital patten of our star moving with all of the planets influence.

This is why I do not think it could ever be possible to have a perfect circular orbit.

It would be very complex