There's no way to upgrade - the CPU is permanently soldered to the system board and there is just one CPU offered for this model - the i3-6100U.
RE: Inspiron 3158 2-in-1 Avaliable CPU upgrades.
It won't -- Intel produces very few mobile processors other than in BGA (soldered) format. The sole remaining ones are the high-end gaming/workstation class chips -- and desktop chips that are also used in high-end mobile systems.
It's just not possible to produce a slim tablet-style system with a socketed CPU.