Click on the icon that looks like this
near the upper-right corner of the window, or choose File > Extension Manager...
You can browse a list of available extensions in the Available tab of the Extension Manager, and click Install to install an extension with one click. Use the filter field to search across the extension name, description, author, and all other fields.

The extension listing comes from the online Brackets Extension Registry. If you're an extension author, please publish your extension to the registry so it's available for others to find.
Under the Installed tab, click on the Remove button to mark an extension for removal. When you close Extension Manager, Brackets will need to restart in order to finish removing the selected extensions. (You can save your work or choose to cancel the restart first. If you cancel the restart, you'll need to reselect the extensions extensions to remove later).
If an extension breaks Brackets to the point where Extension Manager is unusable, see the "Manual" section below to remove the extension.
When an extension update is available, a green Update button will appear in the Installed tag. Updating extensions also requires a Brackets restart (see "Removing Extensions" above).
user folderSome older Brackets extensions are not listed in Extension Manager. These extensions may no longer work in current versions of Brackets.
href and rel attribute urls in editor on ALT+0 shortcut. Currently works only with existing files.