How can I modify mortgage in Chapter 13 bankruptcy?

All you have to do is follow the repayment plan that has been approved by the court. Depending on your income and the debt amount, it may not be possible for you to repay the full amount in 5 years. If you follow the terms of the plan sincerely, the court may discharge the remaining amount after 5 years.

Try to repay the loan as per the repayment plan given by the court.

Yeah, it's actually pretty cool.
Your junior and secondary mortgages will get stripped if your home's value is less then than the loan amount!
Also your overall mortgage amount can be adjusted, if the same thing happens when your value of house is less than the loan amount.

And then when things are adjusted you start making payments for the new loan terms with decreased loan amount. You will have automatic stay on you, so there won't be any foreclosure!

Stick to your payment plan, if you want to get rid of your debts and repay the full amount in 5 years.