The purpose of this study is to find out if the combination of the drugs abemaciclib and letrozole is better or worse than usual approach for your type of cancer. The usual approach is defined as care most people get for endometrial cancer. Another purpose of the study is to test any good and bad effects of the drug abemaciclib when used in combination with letrozole. Abemaciclib and letrozole could shrink your cancer but they could also cause side effects and your cancer could grow while on treatment. Abemaciclib plus letrozole has already been FDA-approved to treat other cancers. There will be about 25-50 people taking part in this study.