Different stages of milk are formulated for different age groups and tailored according to that Recommended Dietary Allowances of that age group. The combination of nutrient levels is different to cater to the needs of children at different stages of growth. It is fine for some mums to continue Stage 2 after their baby turns 1 year old if they prefer. You can take your time to move on to next stage of milk i.e. there is no hurry to change formulation exactly when the child turns 1 as some children may still be sensitive to formulation change. Children has an increased reliance on solid foods as they grow, and growing-up milk caters to children who are already able to obtain important nutrients from other food sources. Therefore, a mother may choose to switch the child when she is 1 years 1 month old and another mother may start when the child is at 2 years old.
You can introduce by feeding one bottle of stage 3 amidst stage 2 bottle feedings. I remember that my pediatrician advises not to mix formula milk powder together so it's better to start introducing 1 bottle of stage 3 first then slowly increase the frequency if your child can adapt to it.