Under the mesh component in Simulation right click and insert a 'Method'. This can be set to hex dominant, which will convert most if not all elements to hex if the program is able. Depending on you're geometry you may also look into swept meshes or other options (check users manual). Let me know if this helps.
First of all, I would like to thank you for your help. The method that you have given to me works but as I can see the elements are not completely hexahedral. In other words, the face of the element is not square. Is it normal and does that affect my analysis?
The elements do not need to be exactly square for an accurate solution, as long as it is not a greatly skewed shape with one or 2 sides more than about 5 times longer or shorter than the other sides. Ansys will generally give you a warning or error message about element aspect ratio if that is the case.