If you are in Fluent Solution mode, you can improve the mesh quality by selecting the improve the mesh option on the Quality button under the Domain tab. To improve the surface mesh, you will have to enter a face quality limit to target, and when improving the volume mesh, you will have to enter a cell quality limit value to target. If you are still in the process of generating your mesh in Fluent Meshing mode, you can insert a task after both the 'Generate the Surface Mesh' task and 'Generate the Volume Mesh' task by right-clicking on either one and inserting a new 'Improve Mesh' task. Mesh quality can be improved in a variety of places in Ansys Fluent.