Three notes on this verse:
First, this verse is calling people who already have a relationship with God. He calls “His people”.
Second, God calls people who already have a relationship with Him to repent. What’s repentance? Turning from sin to God. Why does God say to do it? Because people who already have a relationship with God still sin. Daily. In dozens of ways. A lot of times I think our sin is pretty apparent to everyone else around us, but because we’re so blissfully self-unaware we miss a chance to turn from our sin to God.
Third, God will forgive and heal. Repentance is just another chance to experience God’s grace.
P.S. – If you’re feeling pretty good about yourself because you can’t think of any ways that you regularly sin…. pride is also a sin.