On Tisha B'Av it is prohibited to come in contact with water for pleasure, but it is permitted to wash oneself minimally in order to get rid of dirt. Therefore, it is permitted to wash in preparation for the hefsek taharah. You should minimize your contact with water as much as possible and limit it to the vaginal area only. You would only use soap if you found it necessary for cleaning the vaginal area. If you find this difficult, you could also use baby wipes or hygienic wipes, and then perform a hefsek taharah as usual.

For a hefsek taharah during the nine days, in addition to the above, you could shower in lukewarm water.

In either scenario, you would change to clean white underwear.