She should scrape it, rub it with water, and wash it, and then she may pray in it

Asmā' (may Allah be pleased with her) reported that the Prophet (may Allah's peace and blessings be upon him) was asked about menstrual blood soiling a woman's dress, and he explained how to remove it. He said that a woman should first scrape the affected area to remove the substance of blood. Then she should rub it with her fingertips to wash out the soaked blood. After that, she should wash it to remove the remaining traces of blood. This order should be observed, for this is the best way for removing a dry impurity. After doing this, a woman may pray in such garment. Trying to remove it the opposite way would spread the impurity.