Project Setting

To change the project name or project key, click the name of the project on "Projects" list.

Then click "Edit Project basic settings" on "Project Settings".

Chart

By checking "Use Chart" box, you can use Gantt Chart and Burndown Chart.
You'll be able to input Start Date and Estimated Hours/Actual Hours of issue.

This feature is only available for the plans with chart options.

Subtasking

Checking "Enable Subtasking" box will let you use parent-child issue relationships for this project.

Subtasking feature is only available on paid plans.

Formatting rules

"Formatting rules" is to set the formatting rules for this project. "Backlog", "Markdown" or "Use Space setting" options are available.
Formatting rules for the Space can be set on the Space settings page.

Wiki

Check this box to show Wikis in tree structures.

Project Administrator

In Project Settings, check the box next to Project Administrator to let those users add other project administrators.

Archive the project

Click "Don't display on Dashboard" box if you want to hide this project form Dashboard when the project has been completed, etc.

Changing the project key

If you change the project key, the URL to access issues and Project Home will be changed.
If you use Files or Subversion, the URL of web folder and Subversion repository will change also. Please do not change the project key unless it is not avoidable (i.e. you've assigned a wrong project key).

Please do not change the project key unless you really need to do so.