For the first 90 days, listen and plan - but don't do anything. You need to understand why they are under performing and 9/10 it's because the previous Manager was just not a good Manager, could not recognize people's strength's, had them in the wrong roles, tried to do their jobs etc etc
Read "First break all the Rules" - get to know your team, get to understand their strengths, get people in the right roles (plan a change if reqd) and then your focus after that is break down the barriers that stop your team being successful and get out of their way. “Now, discover your strengths!” that Swier suggests is also a marvellous book\resource to help you in this task.
You are in a very fortunate position - there is nothing more rewarding than turning this situation around and there is only one way to go.... up... and that will get you noticed. It is much harder to take over a high performing team and either keep that going or further improve on it, as it's very unlikely you'll get the chance to really meet the previous Manager and understand why they had success and that team's loyalty will be with the previous Manager - this team will be looking for someone to lead, guide them and help them be successful as no-one goes to work wanting to under perform. Show and help them to achieve that and they will do and achieve remarkable things and you will be so proud of them as you watch them develop, achieve, grow in confidence and keep going. So, give thanks for having landed such a great career opportunity and go enjoy it.