Changing lines are the most important aspect of an interpretation, because these are the points of change into the second hexagram. (When no lines change it usually indicates that the situation will stay the same for a while.) Sometimes the lines contradict what the judgment says, and if you attempt to interpret without realising just how important the lines are for providing the ultimate flavour to the interpretation then you may well misinterpret.
1 line changes
This is your answer, take it to be the answer in preference to a contradictory judgment (the lines always take precedence over the judgment if there is contradiction)
2 lines change
Uppermost line of the two is most important
Middle most important
Go over to the second hexagram and take the lowermost of the two lines that have not changed from the first hexagram.
Example: If you receive hexagram 1 changing in the first four places, you would look at the fifth line of hexagram 20 (the changing hexagram) and regard that as the answer to your question
In the second hexagram take the line that hasn't come from a change in the first hexagram.
Example: If you receive hexagram 1 with all but the third line changing, you would look at the third line of hexagram 15 (the changing hexagram).
The first hexagram's situation is entirely past or on the brink of change, the second hexagram is more important, take the judgment (hexagrams 1 and 2 have an extra line statement that is intended to be read when all six lines change)