Like any kind of building and/or construction, sewing is a lot harder than it looks.
The first question is, what kind of fabric are you working with? Because that makes a huge mofo difference, like building with wood, vs. concrete. It's the fabric that determines what can - if anything - be done.
Second, if you have never worked with a machine, then don't. Like all machines (and it is one of the first really awesome machines at that) a sewing machine allows someone to replace doing something by hand, with the ability to instantly ruin the entire deal. Working with a sewing machine requires incredibility fine (fine enough that most people seem to fail at it, or need to practice for a long time to get it right) hand-to-eye motor skills, with a foot coordination deal tossed in just for giggles. It takes most people a pretty intense study at the beginning to get the basics right.
It's going to be easier to replace the entire strap with a new one then to extend older, shorter ones though, I know that w/o even looking at the garment. I doubt that the straps have enough extra material (that is in good usable shape) to just be able to unstitch and restitch them at one point. Yes, you love the straps, that's were the fun begins. Start going to fabric stores and work at replacing them.
