u^2-2u+3=u^2-2u+1+2=(u-1)^2+2.
The limits of this expression occur when u=1, when the value of the function is 2. The expression never goes lower than 2 because the square term is always ≥0, its minimum value being zero. So the lower limit is 2, but there is no upper limit for the expression by itself. At u=3 the value of the expression is 6. This not a limit for the expression. However, if the question is meant to combine u=3 with the expression as a condition, then the limits are lower limit u=1 (expression=2), and upper limit u=3 (expression=6).