about live events

Hello hari3435  - I'm not able to answer all of your questions but I'll help where I can.  First I will say that there are plenty of videos and resources online that describe how to produce a live event.

how can I change both presenter and my own webcam video during events ?  If you are the owner or producer, you can switch the presenters and their content in the queue before sending the image live to the audience.

   please describe in details . and immediately i have events today 8 pm.

2) some meetings do not  allow people to join as guests . what is this ???  You need to set your settings to allow public attendance and then send the attendance link to whomever you want to invite.

3) how can i take attendance of guests  - https://answers.microsoft.com/en-us/education_ms/forum/all/microsoft-teams-live-event-attendee-list/aec2c57f-6dc6-4c4c-a558-5ced31037f66?auth=1

4) can i use yammer ??  I'm not sure what you're asking here so I'd suggest you search YouTube for the videos that cover Yammer and Teams

5) where is i see that when someone is waiting in the lobby ???  If your event is set as a public event, there won't be a lobby

Teams app compulsory for  Public events   ???  I don't know if it's compulsory but our groups use the app for live events.

sometimes links not work  which I send students - what error messages are they getting?  Is your meeting set for public attendance?

Again, I highly recommend that you review the resources available online.  Good luck with your event.