You have the right to modify any purchased asset. You are forbidden thou, to improve a purchased asset and repack it as yours. You have the right to ask the vendor for additional information regarding functionality, this means how to setup parameters in order to make the asset work or which function to call for a specific effect, but ask how a blueprint works, or why something is the way it is in a C++ code, that’s another story… this is something the TOS does not cover.
Anyway, if you really need to understand how something works, you can start making changes until you figure it out or try to cut a deal with the vendor, like paying for something, but if the asset is just blueprint, all documentation is available, and you might just need to search or use the Answer Hub to obtain the necessary info.
We got a very mixed set of cultures in the community, and sometimes language barriers and cultural ways come into play, be sure to certify yourself that you were really understood before pointing out names or products.
Hope you succeed in your endeavours!