src/Composer/Downloader/DownloadManager.php
public function update(PackageInterface $initial, PackageInterface $target, $targetDir)
    {
        $downloader = $this->getDownloaderForInstalledPackage($initial);
        if (!$downloader) {
            return;
        }

        $installationSource = $initial->getInstallationSource();

        if ('dist' === $installationSource) {
            $initialType = $initial->getDistType();
            $targetType = $target->getDistType();
        } else {
            $initialType = $initial->getSourceType();
            $targetType = $target->getSourceType();
        }

        // upgrading from a dist stable package to a dev package, force source reinstall
        if ($target->isDev() && 'dist' === $installationSource) {
            $downloader->remove($initial, $targetDir);
            $this->download($target, $targetDir);

            return;
        }

        if ($initialType === $targetType) {
            $target->setInstallationSource($installationSource);
            try {
                $downloader->update($initial, $target, $targetDir);

                return;
            } catch (\RuntimeException $e) {
                if (!$this->io->isInteractive()) {
                    throw $e;
                }
                $this->io->writeError('<error>    Update failed ('.$e->getMessage().')</error>');
                if (!$this->io->askConfirmation('    Would you like to try reinstalling the package instead [<comment>yes</comment>]? ', true)) {
                    throw $e;
                }
            }
        }

        $downloader->remove($initial, $targetDir);
        $this->download($target, $targetDir, 'source' === $installationSource);
    }
public function update(PackageInterface $initial, PackageInterface $target, $targetDir)
    {
        $downloader = $this->getDownloaderForInstalledPackage($initial);
        if (!$downloader) {
            return;
        }

        $installationSource = $initial->getInstallationSource();

        if ('dist' === $installationSource) {
            $initialType = $initial->getDistType();
            $targetType = $target->getDistType();
        } else {
            $initialType = $initial->getSourceType();
            $targetType = $target->getSourceType();
        }

        // upgrading from a dist stable package to a dev package, force source reinstall
        if ($target->isDev() && 'dist' === $installationSource) {
            $downloader->remove($initial, $targetDir);
            $this->download($target, $targetDir);

            return;
        }

        if ($initialType === $targetType) {
            $target->setInstallationSource($installationSource);
            try {
                $downloader->update($initial, $target, $targetDir);

                return;
            } catch (\RuntimeException $e) {
                if (!$this->io->isInteractive()) {
                    throw $e;
                }
                $this->io->writeError('<error>    Update failed ('.$e->getMessage().')</error>');
                if (!$this->io->askConfirmation('    Would you like to try reinstalling the package instead [<comment>yes</comment>]? ', true)) {
                    throw $e;
                }
            }
        }

        $downloader->remove($initial, $targetDir);
        $this->download($target, $targetDir, 'source' === $installationSource);
    }
Updates package from initial to target version.

@param PackageInterface $initial   initial package version
@param PackageInterface $target    target package version
@param string           $targetDir target dir

@throws \InvalidArgumentException if initial package is not installed