Expand Raspbian filesystem
The first option available in the raspi-config tool is Expand Filesystem. By default, the size of the Raspbian root file system is 2GB. If you have an SD card with more capacity, the portion of your disk space will be unused. This option enables you to expand your Raspbian installation to fill out the rest of the SD card, giving you more space to use for files.
To expand the root filesystem, highlight the Expand Filesystem option and press Enter:
The process of extending the partition begins immediately; there is no confirmation dialog. You will need to reboot your device. Be patient, since the expansion process lasts for a couple of minutes.