Cut squash into 1" crosswise slices.
Remove seeds and stringy fibers from center.
Arange slices in 12x8x2" (2-qt.) glass baking dish.
Cover tightly With plastic wrap.
Microwave at high setting 6 minutes.
Turn squash over and rearrange slices.
Cover tightly and microwave at high setting 6 minutes more, or until tender.
Combine butter, brown sugar and cinnamon in 2-c. glass measuring cup; mix well.
Microwave at high setting 4 minutes, or until brown sugar is dissolved.
Spoon brown sugar mixture over squash rings.
Microwave squash at high setting 2 minutes.
Baste squash with syrup in bottom of dish.
Microwave at high setting 2 minutes more.