How to treat d-dimer levels that are high normal range below 0.4 and level is 3.9 - has new bruise?

No treatment. D-dimer is a non specific test that can become abnormal in many different conditions. A bruise can cause it to increase. You look for the cause and treat the cause if needed.
D-dimer. This test is often used to evaluate someone with the possible venous thrombosis, but it also can be elevated due to inflammatory issues. It would be recommended to undergo a venous duplex ultrasound of the lower extremities to determine if you have any venous thrombosis based on this elevated level.