I have created a model that uses solid 45 and solid 65 elements, but I don't know how to assign the element attributes for the volumes. For some volumes I want to assign solid 45 and solid 65 for others.
Go to mesh tool, you can select element attributes there. For example, if you have defined multiple materials, you may choose here the relevant one. The choose 'mesh', select the volume to be meshed. This will mesh the selected volume using the chosen set of attributes.