There are 2 ways to promote.
First way: You win the championship of your league. So you need to make the playoffs and then make the finals and beat the team from the other conference. If you do this, you will be promoted to a higher division. So from div 5 to div 4 in your case.
Second way: Quite often there are bots in the division above you. So for example, there might be 100 bots in div 4 in your country. Most of these bots are going to get relegated by coming 8th, or losing the relegation series. However, some bots will not be relegated by these processes, so at the end of the season you might have 12 bots in div 4 in your country remaining. Teams from division 5 will then promote and take the place of this bot team. Which teams get to promote in this way is determined by how you went in division 5. The more games you win the higher your chance. Someone who has won 22 games and lost 0 in division 5 that season has a better chance than someone who has won 12 games and lost 10 games.
Hope that makes sense.