@implor – thanks for your post. I would like to move it to the appropriate product community so your question will have greater visibility. Which product is this in reference to?
You can always try "Select All" (make sure all layers are visible and selectable, as well as all objects) and mirror them.
Assuming you are using PADS Standard:
No, and you can't "mirror" everything. Only drafting items can be mirrored, so you will need to move or renumber the pads in the decal if you want it to be correct in the library.