The Astra automobile goes by different names in different markets. In the UK it's called the Vectra, while in China it's the Buick Excelle. In the U.S. it's the Saturn Astra, which ceased production in 2009. The general mechanical layout of each brand of Astra is similar, including the alternator. If you ensure the replacement alternator is compatible, changing it is a fairly straightforward job.
Put on lightweight gloves before changing your Astra alternator. The gloves protect your hands as the area around the alternator is likely to be greasy.
Disconnect the clamp on the end of the black battery cable that's connected to the Astra's battery using a wrench.
Remove the cables and wires connected to the Astra's alternator. There are two cables: one red, the other black. If the cables have plastic plugs, unclip them and pull the plugs out from the sockets. If the cables are held in place using a nut, remove the nut from the terminal poles using a wrench and lift off the cables. The third connection is a multi-wired plug. Depress the clip on the side of the plug and pull the plug out using your fingers.
Loosen the three bolts on the alternator. One is at the front. The second is at the rear and holds the alternator onto the engine block. The third is an adjustable bracket on the front. Push the alternator toward the engine block, then lift off the alternator belt from the pulley wheel and move it to one side. Remove the three bolts holding the alternator in place. Remove the alternator from the engine compartment.
Put the replacement alternator where the old alternator was. Insert the three bolts into the metal lugs and loosely fit on the nuts using your fingers.
Place the alternator belt over the pulley wheel on the front of the alternator. Pull the alternator away from the engine using one hand so the belt gets tight. Keep the tension on the belt with one hand. Use your other hand and a wrench to tighten the nut and bolt on the front adjustable bracket. Check that the belt is tight, once you've tightened the nut and bolt.
Tighten the two remaining nuts and bolts using a wrench. Check that the alternator is firmly in place and can't move if you push it. Ensure the belt is still tight.
Replace the cable connections. Push the plug on the end of the red cable into the socket labelled "B" or Bat" using your fingers. Or, if it has a screw, place the eyelet on the end of the red cable over the terminal pole, then replace and tighten the nut using a wrench. Use the same procedure to connect the plug or eyelet on the end of the black cable. The terminal is labelled "Gnd." Push the plug on the end of the multi-wires into the socket. Check that all the connections are secure.
Put the clamp on the end of the black battery cable onto the negative battery terminal using a wrench.