Is buying a car from a showroom in installments through the bank with an interest increase, and registering it in the husband's name, considered usury? And what should be done if the husband refuses to sell it to get rid of its sin?

If the bank purchases the car from the dealership and sells it to you at a fixed price in installments, there is no harm in that, and registering the car in your husband's name does not affect it. The important condition is that the bank owns the car before selling it to you, and that the installment amount is not increased if you are late in payment, because stipulating compensation for delay is forbidden usury (riba). However, if you purchase the car from the dealership and the bank pays the price on your behalf, then collects it from you with an increase, this is impermissible usury. Assuming the transaction is forbidden, the prohibition relates to the liability of the contracting parties, and repentance is achieved through remorse, seeking forgiveness, and resolving not to return to the sin. The prohibition does not prevent benefiting from the car. If accelerating the payment waives the usurious interests and that is possible, then one should hasten to do so.