You can find out how to download Linux Mint:
It is recommended to use the bittorrent download option for best speed and efficiency.
Once downloaded, verify your new ISO image file using checksum.
Anyone can produce fake ISO images, it is your responsibility to check you are downloading the official ones.
Download the ISO image, right-click->"Save Link As…" on the sha256sum.txt and sha256sum.txt.gpg buttons to save these files locally, then follow the instructions to verify your downloaded files.