Meters and centimeters
Open the main map page, the first page you see when you open Yardstick. In the row of buttons under the map, click Units: ft. It changes to Units: m, and distances and areas on the page switch to meters and square meters.
The map editor follows it
The map editor has no units switch of its own. It uses whatever the Units button on the map page is set to. Set that to meters, then open the map editor, or reload it if it was already open.
Zone size grows and shrinks by 1 cm
With a zone selected, the Zone size buttons read − 1 cm and + 1 cm instead of 1 inch.
Sizes are shown in meters
The zone's area is in square meters, and its width and length in meters.
Celsius
On the map page
Click Temp: °F, the button beside Units. It changes to Temp: °C.
In the weather panel
The weather panel has its own setting. Open Settings, find Weather, and under Units choose Celsius. That gives you °C, km/h and mm. Press Save.