You find out who Satoshi is (private investigator) , and you hack their computer. Since they invented bitcoin there's a good chance that they own a nontrivial percentage of all currently mined bitcoins.

Likewise you could hack the computers of early adopters who own a lot of bitcoins (back when mining was easy). For example, someone paid someone else 10,000 bitcoin for a pizza a few years ago.

You now own a significant portion all all the bitcoins.

Alternatively, hire security professionals and finds bugs in the bitcoin protocol or client software. You may be able to force another fork.