To view the packages that you are able to upgrade, select the "Upgradable Packages" view from the selector to the left of the search bar. This will bring up a list of the packages where Portage's recommended version differs from the currently installed version. Note that if you have not selected "Upgradeonly" in the "Settings" menu, this may also include packages that will be downgraded to a lower version if they are selected.
If you are happy with the selection of packages to upgrade, clicking the "Upgrade" button in the toolbar will add the selected packages to the Terminal Window emerge queue, with appropriate emerge options.
By default, all packages in your "world" file (i.e. those you have explicitly installed) will already be selected for you, while the dependencies are not. clicking "Upgrade" with this selection would be the same as running "emerge --update world". If you wish to select all or none of the available packages, right-click somewhere in the package list and choose "Select All" or "De-Select All".
If you just want to upgrade everything on your system, it is sometimes best to run a custom command in stead. Some possibilities are: