1. The problem statement, all variables and given/known data
When an object is submerged or floats in a fluid, the..
Pressure on bottom surface > Pressure on top surface
So the resultant force is acting upwards.

Why the difference in pressure? Must both, the top and bottom surfaces, have the same area of contact with the fluid? Since P=F/A ?

A sealed cylindrical steel can is submerged in water. What is the origin of upthrust that acts on the can?

^ This statement does not states the origin of upthrust...Why? It seems right ! Feeling confused 2. Relevant equation
P=F/A
P=hρg

Yes, but a pressure applied over an area results in creation of a force, F = P * A.

What you seem to be missing is information about Pascal's Law, which Doc Al provided in mathematical form in Post #2, and also Archimedes' principle and the concept of buoyancy, which is another name for this upthrust.