Altium - Defining top or bottom layers as a plane layer
This is possible, when in PCB mode press d then k you can then edit the layers as such but you would change the lower layer to an internal plane



You don't specifically need to change the lower layer to an internal plane, a better way is to use fills or polygon planes for the lower layer and keep its definition as a signal layer. This is more appropriate to keep the power planes on one layer (and that way you can have multiple) whereas a internal plane can only have one net attached to it (although you can break these up with a 'negative' trace), much better to use planes.