Kinetic energy of an electron in an orbit in Bohr's model for a Hydrogen atom is given by: E = 13.6/n² eV
de Broglie wavelength of an electron is given by:
λ = h/p
But, p = √(2mE)
λ = h/√(2mE) => λ ∝ 1/√E
Hence, when an electron jumps from the excited state to the ground state, its kinetic energy (E) increases, and therefore its de Broglie wavelength (λ) decreases.