If you want to make changes to a task, click on the task name. This will show all the task details.
The Next steps at the top provides useful pointers on what to do next.
The task has lots of slidy section which you can open by clicking on them. The task name and description are in the Task definition section.
Click on Change details to open up a panel to change the name and description.
Click on Save changes to save changes, and Back to return to the plan view.
Use the same approach to set dates for task.
As well as entering more task details, you might want to change the order of tasks, and to say that some are sub-tasks of others. From the plan view in the activity panel, you can drag tasks around by the sequence number to resequence them, and indent some tasks inside others.
You can also use the New sub task option from the task : menu to create sub tasks within a task, without dragging them around.