1. Download iso file;
2. use imagewriter which is a default software of mint to write iso to usb stick;
3. boot from usb and enter the mint system to install
4. start installation. Choose partition where the old linux system is located. This partition has to be formated with ext4 file format. root directory choose /
5. continue.
if booter is not successfully installed, the system can not be started with information :unknown file system:
then enter the os in usb and install boot-repair to fix this problem:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install boot-repair