How do I update from the Legacy Software to 64bit?

Legacy Software as alerted by your Mac is most likely non-Apple software that is 32-bit only.

If you look in Apple Menu -> About this Mac -> System Report -> Software -> Applications you can see a list of applications and sort them by the 64-bit column to see the 32-bit ones.

You then need to contact each app supplier - perhaps by visiting their websites and look to see if they have newer 64-bit versions. Some really old software will have been discontinued and there will be no new 64-bit version available in which case you will have to find an alternative.