to meddle in / with sth
which means to become involved in sth that doesn't concern you or to touch sth in a careless way, especially sth which isn't yours or when you don't know how to use it correctly.
Example: He had no right to meddle in her affairs.
Somebody has been meddling with his computer.
Besides, you can use the noun meddler (for a person who tries to get involved in sth which doesn't concern them, also called busybody).