The short answer? You can't.
The long answer? There are many methods, like the angion, the jelqing, the bathmate, the injections, etc, that do have effects, but not what you actually want.
The skin can be stretched a ton. It's just skin and you can stretch that. But for the spongious and cavernous tissue inside the penis? You can't.
You can create temporary inflammation (jelqing works well for that) by doing tons of micro damage to your penis which makes it bigger (think of your hand doubling in size after getting stung by a bee), but it doesn't stay that way after healing.
There are no way to create new cells in your penis, you are stuck with the amount of cells and tissue you natively have (if you've reached maturity).
If you have a small penis to the extent that it becomes a handicap, surgery does exist. It's not the easiest surgery, but it's a thing.
PS: If you're fat, lose the fat. Not only does extra fat has estrogenic effects that have a negative effect on your penis size, but the fat pad also makes you lose external penis length. Half of the penis is already inside the body of a normal man, you being a fat fuck make things worse.