MPa stands for megapascals, a unit of pressure. A kilonewton, abbreviated kN, is a unit of force. However, by combining kilonewtons with a unit of area, such as millimeters squared (to get kN per mm^2), you have a unit of pressure. If you need to inflate an object, such as a car tire or air mattress, you can use it to make sure you have the right pressure: you may have to convert from one unit to another by hand.
Multiply the number of MPa by 0.001^2 kN/mm^2 per MPa to convert to kN/mm^2. For example, if you had 23 MPa, you would multiply 23 MPa by 0.001 kN/mm^2 per MPa to get 0.023 kN/mm^2.
Divide the number of MPa by 1,000 MPa per kN/mm^2 to convert to kN/mm^2. For example, if you had 23 MPa, you would divide 23 MPa by 1,000 MPa per kN/mm^2 to get 0.023 kN/mm^2.
Confirm your answer using an online pressure units conversion calculator.