These are some tips that could help you and your older child adjust.
1) Talk to your older child about the time they were a baby themselves, and explain how they grew in your uterus and then you gave birth to them. This is interesting to most toddlers and preschoolers, who love to hear all about themselves, and learn more about how they grew. This approach might not work for older kids, who already know how babies are made and are embarrassed talking about it.
2) Read books about pregnancy, and about getting a younger sibling. Talk about how wonderful siblings can be, but make sure not to tell your child that they will get a playmate, because newborns are cute, but not really fun playmates let's be honest about it!
3) Involve your older child in the preparations for the new baby as much as possible. Let them choose a few outfits, and let them help wash and organize newborn supplies like clothes, blankets, and the like. In my experience, letting an older child help as much as possible with a new baby really helps them find their new role in the family, and can make them feel all "big kid" like.
4) If at all possible, spend some one on one time with your older child even after their little brother or sister has arrived. Of course, newborns are very mommy-focused and need to breastfeed very frequently. Still, even 15 minutes of your undivided attention can make the world of difference to your older child. The earlier months of your new baby's life are also an excellent time for your big kid to bond with daddy.