Get the Source Package from http://typo3.org/download/ and upload this package to your web server. Put it one level above the document root.
For this manual, we will use the .tar.gz file. Use the shell to execute the according commands:
/var/www/site/htdocs/$ cd ..
/var/www/site/$ wget get.typo3.org/7.6 -O typo3_src-7.6.x.tar.gz
Unpack the typo3_src-7.6.x.tar.gz file on your web server:
/var/www/site/$ tar -xzf typo3_src-7.6.x.tar.gz
Create these symlinks in your document root:
cd htdocs
ln -s ../typo3_src-7.6.x typo3_src
ln -s typo3_src/typo3 typo3
ln -s typo3_src/index.php index.php