The safest way to upgrade is to use the action 'safe-upgrade' as the previously used action 'upgrade' has been deprecated.

The new action will upgrade a package only if it does not impact other packages, i-e: if it is necessary to add or remove a dependency package during upgrade, safe-upgrade won't upgrade.

aptitude safe-upgrade

If you want to force the upgrade anyway, the action 'full-upgrade' (previously known as 'dist-upgrade') can be used.

With this command, aptitude will upgrade to the latest version of a package even if it is necessary to add or remove dependencies.

aptitude full-upgrade