That’s a skinning issue and can’t be fixed inside UE4. If you have the original model file, open the file inside your 3D program, Max, Maya, Blender, where ever you exported from, and fix the vertex weighting on there and then export it again.
If this is a “bought” model and it’s an fbx file only, or something like that, you can import it into any of those before mentioned programs to fix the skinning. You probably should also get in contact with the seller and let them know that the model has skinning problems.
If you don’t know how skinning works, it’s good to learn it if you’re working in UE4, so you can search some tutorials on it based on the software package you use and you’ll probably find much more than you need on the subject. In a program like Max, you can also simply look at the help docs, they cover the basics for every tool and should give you a good start on things. To fix this issue you really only need the basic knowledge.