How to update single value inside specific array item in redux

You can use map. Here is an example implementation:

case 'SOME_ACTION':
   return { 
       ...state, 
       contents: state.contents.map(
           (content, i) => i === 1 ? {...content, text: action.payload}
                                   : content
       )
    }

You could use the React Immutability helpers

import update from 'react-addons-update';

// ...    

case 'SOME_ACTION':
  return update(state, { 
    contents: { 
      1: {
        text: {$set: action.payload}
      }
    }
  });

Although I would imagine you'd probably be doing something more like this?

case 'SOME_ACTION':
  return update(state, { 
    contents: { 
      [action.id]: {
        text: {$set: action.payload}
      }
    }
  });