unscrew the screws that hold the blades on , remove and replace.
WARNING ....... the pins that hold the heads on are sometimes loose and can fall out... be very careful when removing the blades that they do not fall out. If they do fall out .. they are very small and hard to find (i know i've done it before). They will go back in as the blade holder is slotted to guide the pin back in. Slide the pin in to the slot while lightly squeezing the blade holder arms together. Use a very fine blade or screwdriver flat head to guide it in to place.. its very fiddly but it does work eventually.
Replace the battery
theres a small panel under the helicopter , simply unscrew the screw and unpoug the battery , reverse to get it back in again.
Anything else you need to do requires removing the body
Remove all screws from the side of the body that has the most screws (NB - blade screws and body screws are different .. do not mix them up).