Chipping away the loose plaster would be the best place to start. Make sure there is no damage beneath the surface of the plaster, If you do find damage, You need to repair it as soon as possible. Then you can begin the plaster repair. The nearest lumber company can usually help you find the right materials. Avoid the premixed patching compounds.
After the repair is finished and dried, you may need to go back and caulk the edges of the plaster where it meets any other materials, wood, aluminum, etc.
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