how to install Mint 18 over Mint 17 on dual boot with Windows 7
 

I have dual booted Linux Mint 17 a couple of years ago alongside my Windows 7. I now want to install Mint 18 in place of 17 but when I get to the 'Installation Type' window I cannot work out which option to choose. The first installation window says I have the option to wipe the disk completely or install alongside the other 2 operating systems so I chose the 'Other' option but at the next window I am confused and not sure what to do to make Mint 18 install over Mint 17. What do I need to choose? Do I just have to change to the Linux Mint option in the dropdown list? I don't want to keep the old Mint17. I have added a photo of the screen that confuses me. can someone tell me what to do?

The screen in the image you posted only shows one Linux partition, sda5 so that is what you need to select to install to by highlighting it in that windows. Click the format box on that line to overwrite the system and install the new one. This will also overwrite any personal data on that partition so back that up to another drive before continuing.

If you are using the Grub bootloader to both Mint and windows and want to continue to do so and you are using a default MBR system with windows, leave the Device for bootloader installation as is.

This article might help. It describes how to do an in-place upgrade.

https://community.linuxmint.com/tutorial/view/2316

Full disclosure: I have not tried this as I am no longer running Mint. I have no complaints; I just wanted to try something different.

/dev/sda is the whole hard drive.

The partition you want on that drive is /dev/sda5 because it is Type ext4 and Linux Mint. If you format that one you will lose Linux Mint 17 and everything else you had with Linux Mint 17. If you mess with the type NTFS partitions you will wipe out Windows so leave those alone.