@frus88 – you can do that with Adobe Acrobat Pro and (depending on the type of image) PhotoShop or Illustrator.
Better, of course, would be changing the original files (InDesign?).
When using Acrobat Pro you could use the TouchUp object tool: just select the object you like to change, right click and chose: "Edit Object…". The selected object will be loaded to PhotoShop (pixel based graphic) or Illustrator (vector based object). After editing the object, just save and close the file without renaming it. The changes will be applied in the PDF.
Uwe
