Changing BPM mid-song?

Novice here at step-mapping. Using ITGmania's editing tool to modify a song. However, the song speeds up toward the end and it's hard to map the steps on beat because the beat changes.

Can someone explain to me like I'm an idiot how to increase the bpm of the song midway through? I know it's possible because someone did a great job of doing that on the Rush-E song.