As described already in a different post in this page, each Kumari selected based on different tests she must pass, including astrological suitability.
The living goddess is living in a palace in Durbar Square in Kathmandu and she will stay there till she get her first period and then another Kumari will be selected.
New Kumari – Daily Mail UK
According to Hindu mythology, Lord Indra, King of Heaven, rode on his elephant and met the sage Durvasa, who offered him a bouquet of flowers he had received from Shiva. Indra accepted the bouquet and put it on the elephant to show he was not an inconsiderate god. The elephant, who knew Indra, knew that he was not as he pretended to introduce himself, but the opposite, his ego guiding him. So, he took the bouquet and threw it to the ground.