I have installed Mint 17 on a disk which had been in use on 32 bit. Naturally it has installed as 32 Bit. However I would like to move to 64 Bit. If it requires a complete reformat can anyone tell me how I can find and use a format command?

I'm not sure what you mean by "I have installed Mint 17 on a disk which had been in use on 32 bit". Are you saying you had 32-bit Mint installed on a hard drive in a 32-bit system, then you removed the drive from that computer, plugged it into a 64-bit machine, and now want to convert your Mint installation to 64-bit?

If that's the case I believe you'll need to reinstall Mint to convert, just run the 64-bit installer and reinstall like you did the first time. Note that you'll likely lose all of your data and settings unless you put them on separate partitions, so you'll want to make a full backup first so you can restore the necessary files.

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Just a quick note that if you eventually decide to re-install Mint then, unless you have some overriding reason not to, you may as well install the current version, Mint 18.1, downloadable from the Linux Mint website.

You haven't mentioned whether you have verified that the device has 64-bit architecture. That's prerequisite for running 64-bit software. You can do so by running

If it is 64-bit architecture, you will need to back up any crucial data and install 64-bit Mint from scratch. The installation system includes a format routine.

See man uname for more about the uname command.

The "32-bit" description refers to your processor and the software that it can run. Disk format is the same, regardless, so installing a 64-bit distro is just like installing a 32-bit one: the disk partitioning doesn't have to change.

Indeed!

You should also note that a 64bit OS is capable and will often need to run 32bit software or drivers, but a 32bit OS cannot run 64bit software.
The 64bit world is still catching up as far as supported software is concerned.

It is also worth saying that unless you have more than 4gb of memory or need the higher addressing capability there is little point in changing and indeed performance is likely to suffer for day to day tasks

I'll add my confirmation that you would need to reinstall the 64-bit version from scratch - it is not possible to "convert" a 32-bit installation of an operating system to a 64-bit installation. Take note as well of the other comments above concerning the amount of memory, performance and disk formatting (unrelated).

Cheers,