I have been unable to find a solution online for the problem I am having. I defined a matrix [A:=] with the units of psf. When I restate the matrix with an evaluation [A=], the units are psi. How do I change the units from psi to psf in the evaluated expression? The matrix contains strings and constants (defined with units). I am using an older version (Express Prime 126.96.36.199).
I'm afraid there is no solution for your problem.
For a single value, you can set ('force') the value to a specific unit, by putting that unit in the placeholder of the result. While Prime allows multiple units in a matrix, there is no way to force the matrix output to another value, unless it applies to all matrix elements. In your matrix that's not the case.
As Luc already wrote, there seems to be no satisfactory solution for your problem.
There is no way to force the unit in the display of a matrix for single elements. We can only change it for the whole matrix and this does not work in your case because of the text-strings.
Furthermore Prime does not provide any mechanism to change the default unit in a given unit system.
If you display a matrix with the values only without the row and column headers, you can force them to be displayed in psf (because all matrix elements are of the same unit.
To create a table with headers, the only workaround I can think of is to divide the values by the desired unit and note that unit in the header description:
This would also work with quantities of different units, but it shouldn't be necessary to do so given we are using a software claiming to be able to deal with units in a natural way.