The Y_1−xTb_xMn₆Sn₆ compounds (x=0, 0.175, 0.2, 0.225, 0.25) were studied by small angle neutron scattering (SANS) and paramagnetic neutron spin echo. The YMn₆Sn₆ compound is found to be a helimagnet in the whole temperature range below T_N=310 K. Close to T_N an additional peak of a Lorenz shape was observed at Q=0. The peak is thought to have originated from the ferromagnetic fluctuations of the magnetic Mn moment in the ab-plane of the hexagonal crystal structure. Compounds, in which Y is replaced by Tb, change their magnetic order with the increase of temperature: from easy cone ferromagnetic phase at low T through the helicoidal phase to the ferromagnetic fluctuation close to T_N. Temperature-concentration phase diagram of Y_1−xTb_xMn₆Sn₆ is built on the basis of the obtained data.