If your community is a contribution phase such as "Brainstorming or Research", then you can add a contribution either by clicking the big phase idea button on the start page or under My account > New Idea / Research. Depending on the phase your community is at the moment, you can or cannot create contributions.
You either go to your profile page ("My Account > my dasboard"), where all your contributions are listed and click the edit button, or you go the respective idea and click the edit button next to the title.
You cannot delete an idea by yourself. You need to contact the moderator of your community.
If the administrator of your community has turned on the Wiki feature, then all users can contribute to your idea and change content. But all changes will be saved and you can see the history of changes and easily turn changes back if necessary.
Yes. Each idea can be a text collaboration space, where all users can add and change content of an idea. All changes are saved as revisions in a history. So you can easily can go back and see who changed what.
You can edit your comment any time. Just go to the comment you want to change and click the button as shown below. For each a date will be added, which shows when the comment was last updated.
No. You need to contact the administrator of your community for that. But you can update your commnt at any time clicking the edit button next to the comment.