You already know how to use trapz. Just use it twice. However, you mention instantaneous displacement, and the problem with trapz is it gives you an integral over the entire range.
So if you look at the help for trapz, you will see at the bottom, the name cumtrapz.
Cumtrapz gives you the integral cumulatively up to that point. I think this is what you are looking to find. Again, just use cumtrapz twice. The first call gives you velocity. The second call is dosplacement.
In general, if you don't know how to do something in MATLAB, then use tools like help, doc, lookfor. Check the links at the bottom of the help, as they may point you to something good. For example, if you tried this:
then it would have mentioned cumtrapz as a possible utility of interest.