Due to wide spread usage of smart phones, there is a rapid increase in the demand for high data rate services from users in a cellular network. With users undergoing multipath fading and shadowing, and base stations facing transmit power constraints, several users may face outage. In this paper, we deploy an unmanned aerial vehicle (UAV) as a decode-and-forward (DF) relay in a cellular system, which helps base stations (BSs) in providing desired rates to the users. Due to the channel reuse policy in a cellular network, the UAV while forwarding the information to a user, may interfere with other users of the same channel, leading to inter-cell interference (ICI). Considering the number of user equipments (UEs) in outage (UEiO) as the objective, we propose a particle swarm optimization (PSO) based solution to find the best location of the UAV which minimizes UEiO and keeps a check on ICI. For comparison, we evaluate the UEiO when the location of UAV is fixed and not optimized. Our simulation results demonstrate promising decrease in UEiO.