1/f = 1/ u + 1/v
1/v = 1/f - 1/u => v = u f / (u - f)
in a concave mirror with an inverted real image on screen, all quantities, v , f and u are negative. So the negative sign cancels out.
if u increases a little, then v decreases, If u decreases then v increases.
Depending on the position of the object and screen with respect to the mirror, one may have to move in either direction.
Moving the mirror away or towards the object , could depend on the position of the object -- if it is outside the 2 * focal length distance or if the object is placed between the center of curvature and the principal focus point.