bus or train from kunming to Beijing

Only two direct trains are in service between the two cities each day - T62 and K472. The journey takes one day plus thirteen hours and one day plus twenty-two hours respectively. So you can get to Beijing in two days, but it would be very tired.
Anyway, the ticket costs 320 RMB for the hard seat, 540-580 RMB for hard sleeper and 852 - 890 RMB for soft sleeper.

And no direct long-distance buses are in service between the two cities.
While I do advise you catch a flight, it only takes about 3 hrs to Beijing. The flight ticket fare is at least 1,670 RMB.

If you want to book the railway ticket, the website may help you:
https://www.travelchinaguide.com/china-trains/
Similarly, you can also book the air ticket on TravelChinaGuide. com