Is it permissible to rub underwear that has become impure with urine with water to purify it for prayer, or must it be replaced?

If urine gets on clothes and you want to pray in them, the affected area must be washed thoroughly with water until the impurity is completely removed. There is no need to change the clothes, and prayer in them is valid after washing them, based on the Prophet's (peace be upon him) hadith regarding the washing of menstrual blood.