  We show that a heavy fermion superconductor UPt_3 is a topological Weyl
superconductor with tunable Weyl nodes. Adopting a generic order parameter in
the E_2u representation allowed by nonsymmorphic crystal symmetry, we clarify
unusual gap structure and associated topological properties. The pair creation,
pair annihilation, and coalescence of Weyl nodes are demonstrated in the
time-reversal symmetry broken B-phase. At most 98 point nodes compatible with
Blount's theorem give rise to line node-like behaviors in low-energy
excitations, consistent with experimental results. We also show an arc node
protected by the nonsymmorphic crystal symmetry on the Brillouin zone face.
