Changing the Top Value

Double Click on the top scale value on the Y axis scale on the Line Graph.

Enter the new  value and click OK.

Changing the Bottom Value

Double Click on the bottom scale value on the Y axis scale on the Line Graph.

Enter the new  value and click OK.

Changing Both Top and Bottom Values

Double Click on the midway area on the Y axis scale on the Line Graph.

Enter the new  values and click OK.