To create a new synthetic variable to perform a unit conversion, do the following:
Navigate to the device that you wish the variable to be in.
Scroll down and click Add Variable
and then select Synthetic
Enter the expression required on the left. To reference other variables, find them in the list on the right and click them to put them into the expression. (Here we see a conversion for wind speed from mph to metres per second)
The window will check to see if the expression is valid and will show a green tick when it looks like it is a valid expression.
When you are happy, click Accept (this button will only be available if the expression works).
You can then name the new variable and set the units for it etc.
There are many more possibilities for synthetic variables, see the manual for more details.