What is the ideal treatment for a 55-years-female with Simple hyperplasia of the endometrium with Atypia?
Management of Atypical Endometrial Hyperplasia:
"Hysterectomy is the best treatment for women at any age with atypical endometrial hyperplasia because the risk of concurrent subclinical invasive disease is high."
Williams Gynae 1st ed p 691
"In presence of atypia, response to progesterone therapy is poor and the relapse rate is high. Nearly one-third of them will progress to cancer and one-fourth may already have associated undiagnosed cancer. In women approaching or post menopause, hysterectomy is a safer choice in those with complex or atypical hyperplasia."  
Jeffcoate 7th/ ed p 425
So Remember: Best management of atypical hyperplasia is hysterectomy.
Also, Know Management of Endometrial Hyperplasia:
Management depends on the patient's age and the presence or absence of cytological atypia.
Non-atypical hyperplasia:

Premenopausal women: Progesterone therapy-

options: - Medroxyprogesterone acetate for 21 days a month daily for 3 months.
Progesterone-containing IUCD.
Postmenopausal women

Simple hyperplasia without atypia - Generally followed without therapy.
Complex hyperplasia without atypia - Cyclical/continuous progesterone therapy.
These patients should be followed annually by endometrial biopsy.


Atypical hyperplasia:

The ideal treatment is a hysterectomy
Premenopausal women willing to preserve fertility - High dose progesterone therapy after full information of risk of undiagnosed cancer or progression to cancer. In these cases periodic TVS and endometrial biopsy is necessary.
Simple hysterectomy
Medroxy progesterone acetate (MPA)
Levonorgesterol (LNG)
IUCD