Don't know what i should do with my undescended testicle?

Depends on location: At your age not worth doing anything if testis is palpable ; doesn't bother you. Worth removing, with or without placement of testicular prosthesis if causing pain or discomfort. 1 testis will suffice for both testosterone ; sperm production. Undescended testis at age 26 years won't produce sperm. Consider minimally invasive laparoscopic removal of testis if intra-abdominal to avoid cancer risk.