High performance quantum networks require extensive utilization of multiplexing techniques, relaying on photonic spatial and temporal structure. Multimode quantum memories for light, where photons are stored in the form of collective atomic excitations, may constitute building blocks of such networks; however, a requirement arises to implement complex linear operations on the stored excitations. In our photonic processor we experimentally demonstrate how the off-resonant ac-Stark shift (ACS) may be employed to manipulate the spatial and temporal structure of photons stored in a highly multimode quantum memory as the collective atomic excitations - spinwaves.