Topological phases of matter is an active research area of condensed matter physics. Among various topics, the symmetry protected topological (SPT) phases, which are generalizations of topological insulators, have attracted enormous theoretical interest in the last few year. Much progress has been made in classifying and characterizing SPT phases. Yet another interesting topic is to study the phase transitions between different SPT phases. In this talk, I will present a lattice model for (2+1)D bosonic SPT phases, which is free of the sign problem and can be explored by quantum Monte Carlo simulations. It turns out that the model demonstrates a rich phase diagram with exotic topological quantum phase transitions. In particular, I will focus on two of these transitions and report our on-going progress to understand them both theoretically and numerically.