One of my favorite things about teaching a pilates session is empowering the student to know how to change their own equipment springs, particularly on the reformer.
Changing springs between exercises is part of the transition between exercises, and transitions are part of the work out. Practicing transitions is just as important as practicing your exercises.
When you change springs on the reformer, the way you change your springs depends on exercise you were just doing. For example, if you have just finished the hundreds, you change your springs in this way: 
1. Take both handles in one hand.
2. Lift your head and use your abdominals to come up to sit. On your way up, spin to the side. You should be sitting, turned to the side, without letting your feet touch the ground. As you feel more and more comfortable coming up to this position, try to spin to the side and sit in a teaser position with your legs. 
3. Once you are sitting to the side in teaser, use your free hand to change your springs.
If you are just finishing an exercise like Elephant, and you were standing on the mat, change your springs by kneeling one or both knees down on the mat.
If you are standing on the floor next to your reformer, change springs by lunging forward towards the springs (bending the front knee and extending the back leg) or squat down, bending both knees. Either way, use your core and inner thighs to lift yourself back up after you change springs.
Notice... that you are not instructed to sit on the carriage, straddle the mat, and change springs between your legs. And you are not asked to stand next to the reformer, bend down with straight legs, and change springs. These positions/habits are terrible for your back and it makes me cringe just thinking about it!
Please do not try to change your springs in ways that could damage your back! It is a waste of time, bad form, loss of attention, and... I will just make you do it over again. :) Read more!
Changing springs between exercises is part of the transition between exercises, and transitions are part of the work out. Practicing transitions is just as important as practicing your exercises.
When you change springs on the reformer, the way you change your springs depends on exercise you were just doing. For example, if you have just finished the hundreds, you change your springs in this way:
1. Take both handles in one hand.
2. Lift your head and use your abdominals to come up to sit. On your way up, spin to the side. You should be sitting, turned to the side, without letting your feet touch the ground. As you feel more and more comfortable coming up to this position, try to spin to the side and sit in a teaser position with your legs.
3. Once you are sitting to the side in teaser, use your free hand to change your springs.
If you are just finishing an exercise like Elephant, and you were standing on the mat, change your springs by kneeling one or both knees down on the mat.
If you are standing on the floor next to your reformer, change springs by lunging forward towards the springs (bending the front knee and extending the back leg) or squat down, bending both knees. Either way, use your core and inner thighs to lift yourself back up after you change springs.
Notice... that you are not instructed to sit on the carriage, straddle the mat, and change springs between your legs. And you are not asked to stand next to the reformer, bend down with straight legs, and change springs. These positions/habits are terrible for your back and it makes me cringe just thinking about it!
Please do not try to change your springs in ways that could damage your back! It is a waste of time, bad form, loss of attention, and... I will just make you do it over again. :) Read more!