When a new baby joins a family it's a big adjustment. That adjustment can be especially challenging for a preschooler. She now has to share you and your attention and that can be confusing to her. You're so busy with the baby that you may not be spending as much time with your preschooler.
There are some things you can do to make the adjustment easier for your preschooler.
When the baby is napping or being cared for by someone else, spend some one-on-one time with your preschooler. Even if it's only fifteen minutes it will give your child the attention he is missing.
Let your preschooler help care for the baby. She can hand you diapers, wipes, etc. Ask her opinion such as "Do you think the baby would like to wear the blue shirt or the green one?"
Reserve an activity for your preschooler to do only when you're feeding the baby. That might involve working puzzles or coloring. You could read him a book while he turns the pages.
Sometimes preschoolers exhibit regressive behaviors. It might be their way of getting your attention. Allow your child to have special moments when they can revisit their days as a baby. Look at old photos or videos of him as a baby. Enlist his help in making his own album with duplicates of those photos.
With time and some help from you, your preschooler will adjust to and accept the new baby in your family.