Blazer top sag / repair

[oldrustytrucks]

My blazer has a double wall top. As most I have seen, mine has the typical sag in a couple places. What is the best way to fix this? Also wondering about a gap between my rear hatch and the tailgate. It is not huge but it is present. I might be missing something but why does the hatch lock and the tail gate still open? This might be obvious to some of you with blazer experience, but this is my first so please bear with me! Thanks for the help!
Posted via Mobile Device
Posted via Mobile Device