Hi Meiyee
You are asking questions about the Jungfraujoch tickets offered by Jungfrau Railways. You would have to ask them if you have specific questions related to your Jungfraujoch visit. A quick check by myself in their FAQ came up with this answer:
Can I spontaneously extend or shorten my stay on the Jungfraujoch?
Yes. The costs of reservations already made will not be refunded or credited. When travelling without a seat reservation, waiting times at Kleine Scheidegg or the Jungfraujoch must be expected.
You can also buy the tickets with sbb.ch/en, yet not a seat reservation. This has to be made on the Jungfrau website.
As a personal advice: do not buy Jungfraujoch tickets in advance. In case of bad weather you will end up in the clouds (unless you simply want to experience snow). Wait until you have arrived in the area and you do know the weather forecast for the upcoming days. Weather forecast in the mountains is rarely reliable more than 2 days out. You can also buy tickets at any staffed railway counter, either with SBB or Jungfrau railways.
Best regards,
Roland