United went to halftime leading 2-1 after coming from behind to take the lead through goals from Phumelele Bhengu, who is on loan from Moroka Swallows, and former Mpumalanga Black Aces striker Brice Aka. Thanda had taken the lead in the 18th minute after goalkeeper Modiri Marumo failed to deal with Sipho Made's long-range freekick.
United wrapped up the victory with two more goals in the second half, one from Bengu who completed his brace and the other one came from substitute Lunga Zondi.
In the early match, Leopards condemned Vasco da Gama to the National First Division after winning to 3-1 to advance to the playoff final 4-2 on aggregate.
The loss means Vasco, who finished 15th on the Premier Soccer League last season, only stayed for just a season in top flight football after gaining promotion last year at the expense of Leopards who they beat in the playoffs final.
Vasco were the first to score through captain but Leopards equalised through Edgar Manaka and Rodney Ramagasela followed with a brace to send the Limpopo side to the final.
United and Leopards will contest the first leg of Promotion Playoffs final on Wednesday and the second leg on Saturday, whether the winner will gain promotion to the Premier Soccer League.