Cool stars with convective outer envelope show similar-type of magnetic activities to that of the Sun hence have a similar type of dynamo. Although, the observations of these stars have introduced a range of stellar rotation periods, gravities, masses, and ages, which put into the debate on the magnetic dynamo. The important observational evidences of these magnetic activities are the presence of dark spots on the surface, short- and long-term variations in spot-cycles, and flares. In this thesis, we have chosen five active stars with luminosity class V to III to investigate the evolution of magnetic activities. These active stars are LO Peg, 47 Cas, AB Dor, CC Eri and SZ Psc. Using ~24 yrs long multi-band data, we found that the surface of LO Peg rotates differentially showing solar-like SDR pattern with a period of ~2.7 yr. The surface coverage of cool spots is found to be in the range of ~9--26 % in LO Peg. Several moderate-sized flares and two superflares on active dwarfs 47 Cas, AB Dor, and CC Eri are also analyzed. A large flare on evolved RS CVn type stars was also observed and analyzed. The time-resolved spectral analysis during these flares shows the variation in the coronal temperature, emission measure, and abundances. The peak temperatures in these moderate flares are found to be in the range 10--90 MK, whereas the peak flare temperatures in superflares are found to be mode than 120 MK. In most flares, during the decay, a sustained heating was detected. Using the hydrodynamic loop modeling, we derive loop-lengths of the flares of the order of 10^10 cm. In nutshell, we found that the magnetic activities in these star changes with time.