I can't really understand the question from the way it's phrased, but in sketcher, typing a negative value will flip things around, but it won;t show the negative value. So, say you have a line that leans to the right 30deg and is dimensioned from the right datum plane, by making the angle -30, it'll lean left 30deg.
I don't know what you mean by the "normals" but if you right-click on the sketch in the model tree and choose "Edit", the dimensions will show up. Double-click the dim of your choice to enter a new value, hit enter, and then regenerate the model. Or if you choose "Edit Definition" (PTC: would you please rename that back to "Redefine"?), that will take you back to inside the sketch to do any thing you wish to it.
If the lines were created 90 deg to each other, sketcher will add horizontal and vertical constraints or a perpendicular (normal) constraint. Highlight then delete the constraint in SKETCHER and the intent manager should automatically insert an angle dimension. Replacing the constraint with an angle dimension will allow user to modify the angle of the lines.