You have learned how to edit meshes, but you were doing it in a destructive manner, which means you were transforming the figures directly. Therefore, the only way to return to the original object is by pressing Ctrl + Z until you return to the very beginning.
However, in Blender, there are some ways to edit your meshes in a non-destructive way. That way, you are not changing the original object but previewing changes you want to make. That can allow you to return to the base figure at any time. sounds amazing, doesn’t it? Well, it’s time to learn how to use modifiers.
With modifiers, you can perform many effects automatically that, in other ways, would be too tedious to do manually and without affecting the base geometry of your object. You can add multiple modifiers to any object layer. you can see these modifiers at the Properties Panel on the right hand of your board, just down from where you can see how many objects you have on your scene and is called the Modifiers tag.
So, to demonstrate this more optimally, let's start with the monkey model. Just press Shift + A, select the Mesh options, and pick the Monkey. As you can see, this primitive is composed of straight polygons, and if you want to smooth the shape and subdivide the mesh, you can do so in a non-destructive way avoiding the hard job of editing it directly.
Now, in Object Mode, select the model, go to the properties panel, and select the properties tab. You can identify it by the wrench icon. Next, click on the drop menu Add Modifiers, and select the option Subdivide.
You will see a new window pop up in your modifiers tag, and you will also see how your monkey primitive is subdivided and smooth. Appearances can be deceiving, but in this case, for good. If you go to Edit Mode, you can see that the original shape (including vertices and edges) is still the same as the original object without the smoothness.
You can run as many subdivisions as you like by going into the properties panel at your right and changing the values of Levels Viewport. But be careful, don’t over-increase it because Blender needs to calculate the vertices' new position, so if you add a lot, the program may crash your computer.
Note: The first level viewport will change the appearance of your object dramatically, but the second onwards will change your model a little bit less.
You can quickly toggle the visualization of this modifier by clicking on the computer monitor icon.
Next to that icon, there are other visibility control settings like the square with a highlight vertex that serves to turn on and off the modifier, especially in Edit mode. So, if you have it on, you will see the smooth and subdivided mesh, but if you turn it off, you will see the base shape of your object.
The methods of subdivision control are above the Level viewport. For example, the Catmull-Clark option will subdivide and smooth your figure, but the Simple option will simply subdivide without smoothness.
So, after having done everything you wanted, now you ought to apply it definitively to the base figure. Go to the dropdown arrow on the modifier tag and select “Apply” or just press Ctrl + A.
Note: You can only apply a modifier in Object Mode.
Finally, you can add as many modifiers as you consider in the Modifiers Tag. You can also add two same modifiers but remember: the way you set them will change your object. Also, it is crucial to determine which modifiers are first and then at the end because that order also affects your model. There you have it! Good Luck!