If this were a field of type calculation, changing the value of Repair Type would automatically update the value returned by this calculation. Since it doesn't, you probably have a field of type number with this expression used as an auto-entered calculation. If so, there's a check box: "Do not replace existing value..." that should be cleared so that subsequent updates of the value of Repair Type will cause this expression to re-evaluate and enter a new value.
Suggestion: instead of entering 0.0875 for your tax rate, put this value in a field somewheres. Then, when the government changes the tax rate, you don't have to redefine this expression, you just enter the new rate into the field.