Opencv3.2 can be installed using apt-get. In https://github.com/Xilinx/PYNQ/blob/master/sdbuild/ubuntu/bionic/aarch64/multistrap.config
find all opencv related package (searching opencv keyword), and install them. Something like:
apt-get install libopencv-calib3d-dev
Alternative way is to install from source, using the well-known cmake flow. pip install won’t upgrade any of your opencv package since we did not put opencv in the dependency list in setup.py.