There is no such thing as devolution, except in the form of primal devolution, in which Groudon / Kyogre revert back to their original state when wielding the Blue / Red Orb respectively, but that doesn't really count. The only way to get a Pokemon back to "devolve" Pokemon is through breeding.
To breed a Joltik, you need to breed a female Galvantula with a male Pokemon the bug egg group, or you need to breed a Galvantula / Joltik with a Ditto.