Metronomes are ticking devices, either wind-up or electric, that keep a steady beat at a given tempo (speed). The numbers indicate ticks per minute, so MM=60 means one tick each second. Metronomes are helpful practice tools for both increasing or decreasing your speed in a piece. But, all they do is tick. Your job is to incorporate the pulse into yourself.
Metronomes generally aren't much use until you're well along in your ability to play your piece, since their function is to aid fluidity. That doesn't come until you know your notes quite well. A metronome comes into play when you have struggled with a section and can't get that tricky rhythm, overcome an awkward spot, or conversely, control your headlong momentum in a section that's quite easy, where you might tend to rush.
For complex rhythms: Imagine each tick is the next-to-smallest note value (if your section has lots of 16th notes, each tick is an eighth note). Set the tempo at a speed you can play EASILY. It might be slower than you think! Once you can play your rhythms correctly with the beats, move the speed up ONE increment. When you have fair facility up several increments, then set the metronome at half its present speed, and imagine it as the next larger note value (now it's ticking quarter notes). Continue until you are at the tempo you want.
For awkward passages: First, see the article on backwards practice to ensure you've solved your note and fingering problems. Add metronome when want to increase speed. Set the tick for a speed you can play EASILY, with no errors. It will be slower than you think! You will spend more time at this level than any other, as your fingers learn to play at a steady tempo. Be patient and get it smooth, accurate and comfortable 3 times in a row. Then increase the tick by ONE increment, and repeat the process. If you've done your homework well, the next few increments will be easy, so you can skip the 3 times in a row rule, but NEVER skip an increment.
Eventually, you'll "hit the wall", encounter a tempo that trips you up over and over. You've met the spot at which the brain has to do some major chunking re-organization to go faster, much like the way your body balance changes as you shift from a brisk walk to a run. You may be done for this session. The next day, back up the tempo a couple of levels, and move up again bit by bit. Your subconscious has been working this out all night, and chances are you'll sail right through that wall, eventually hitting another. Back up again a couple of levels. You'll get through it in one more session. In fact, most good practicers find that the section which was the hardest, slowest, worst part of their piece becomes the smoothest.
By never skipping an increment, the tempo changes are so subtle that you "fool" your fingers into thinking that the speed never really increases. But it does!
To slow down a rushing passage: Start about 8 measures before your problem spot. Set the tempo a few increments slower than usual, and play easily into your rushing section. Relax, breathe out, reduce passion as you practice. Then, still at the slower pace, begin to repeat the section, but each time you play it, concentrate on one aspect that you'll increase rather than tempo: more pronounced articulation, more dramatic dynamics, more distinct voicing to name just a few ideas. You'll find you have plenty to do, and glad you have the time to do it! Attention to these expressive devices will channel your energy away from tempo and into color. Increase your speed one increment at a time until you are at the tempo you want, with vivid color and drama, and no rushing.