If I understood you correctly, initially you have a 2D horizontal flow model. 2D horizontal flow models do not have nodal elevations. Instead, the aquifer top elevation as well as the aquifer base elevation are essentially elemental properties within an unconfined architecture. You may see that in the Data Panel.
If you are interested in assigning elevations for a 3D model I suggest to enter the third dimension (Layer configurator) and assign nodal elevations afterwards.