Hello there! It looks like the information from those links didn't come through, but don't worry, I can certainly give you some general guidance on how to fix your sofa springs. A sagging sofa is a common problem, and it's often a very fixable one!
First, you'll need to figure out what's going on under the cushions. Flip your sofa over and carefully remove the thin fabric dust cover on the bottom, which is usually held on by staples. A flathead screwdriver or staple remover will work well for this. Once you can see inside, you can assess the problem. You're likely looking at one of two main types of springs: zigzag "S" shaped springs that run from the front to the back of the sofa, or individual coiled springs.
If you have zigzag springs, the most common issue is that one of the small metal clips holding the end of a spring to the wooden frame has broken or come loose. You can buy replacement clips online or at an upholstery supply store. To replace one, you'll need to unscrew the old clip, screw in the new one, and then carefully use pliers or a special spring-pulling tool to stretch the spring and hook it into the new clip. Be careful, as the springs are under tension! If the spring itself is broken, you'll need to measure it and order a replacement of the same size.
For coiled springs, the problem is usually that the twine holding them together and in place has frayed and snapped. This allows the springs to lean or sag. The fix here is to re-tie them using strong upholstery twine. This can be a bit more complex, but the goal is to create a taught web of twine that holds the tops of the coils level and prevents them from shifting. Once your springs are re-secured or replaced, you can simply use a staple gun to reattach the dust cover, and your sofa should feel much more supportive.
