@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.
Or you can open the PDF in Illustrator and create a new PDF if the previous reply doesn't work out.
If you want to see what created the PDF file itself, look in the document properties inside acrobat. Having the original file will always be the best work flow.
Another option might be to Place the PDF in InDesign, then mask out the unwanted element(s), either by resizing/reshaping the frame to crop, or layering over with new frames filled with [Paper] and/or the replacement element(s).
It's not ideal, of course, but in a pinch I've even constructed whole new layouts re-using and rearranging cropped bits of PDF documents.
All those ideas are great - I don't really like the one about opening the PDF in illustrator - that's opening a can of worms. There could be font issues and text moves or slides behind other things, clipping masks etc. can go awry.
I'd avoid opening in Illustrator unless an absolute last resort.
The Touch Up in the Acrobat Pro is the best bet I think and will probably do the trick.
Other than that masking it in InDesign is a good bet too.
Thoroughly agree that getting the Source Files is best!
If you have can't get the source files then you should recreate the document properly when you get time so the next round of edits is easier.