First you need an accessor to the mesh which is probably a sub component of your actor. To make it point into the correct direction, at least for a straight aim without any delay, get the vector from your turret to the target and use that to create a rotation, which can be added or set to the world or local rotation/translation of the sub component.
For detailed instructions for each step use google/answerhub/forum/wiki, for example: how to get actor mesh component in blueprint/c++.
And to tell you the truth, all this would be much easier/cleaner/shorter if you did it in c++.