Hello Sara Make a plane from the circle at the bottom of the green cylinder. Make a sketch on that place of a circle equal to that diameter. Use the Pull tool to create a short solid rod. Now Pull the top face of that rod and use the Up To button or press the U key on the keyboard and select the large cylindrical face of the solid filter. It should merge. If you get an error message, use the Move tool and move the short solid rod along the X axis slightly toward the center point of the filter. Now try Pull Up To again.
Regards Peter