DCP-o-matic is often used to take content in formats such as MP4 and make it into a DCP. It can also be used to take existing DCPs and modify them in various ways.
The first step in manipulating an existing DCP is to import it. Click Add DCP... and select your DCP's folder. It will be added to the DCP-o-matic project. If the DCP is unencrypted you can preview it in the normal way, though playback may be slow as decoding of DCPs is almost as computationally intensive as encoding them.
If your DCP is a Version File (VF) (i.e. it refers to another DCP's assets) you should import it as follows: