To do a small bust adjustment without affecting the rest of the bodice, draw a line in the centre of the bust dart to the apex. Draw a second line from the apex to the hem, this line must be parallel to the grainline.
Draw a last line from the apex to 1/3 of the armhole.
Cut the line starting from the hem, passing by the apex and ending at the armhole leaving a notch. Cut out the line of the middle of the bust dart leaving a notch at the apex.
Draw a parallel to the line from the apex to the hem of half of the measure you want to reduce. (If you need to reduce the bust by 4cm. in total, draw a line at 2cm.).
Then, place the side piece on this new line, note that the bust dart will overlap, then paste the rest. The middle section is longer than the side, draw a line perpendicular to the centre front.
Cut along this line and align the part to the waist.
Redraw the bust and waist dart. Close the darts and redraw the side and the waist as well as the armhole.