OK, I'll assume you've not changed a back tyre on any bike.
While it is upright undo the quick release for the back brake, it's not easy or obvious on the Wisper and might be a good idea to loosen the cable at the back brake.
Put it in top gear and take all the air out with the bike upright, the weight helps.
Loosen the 18mm* wheel nuts and turn the bike upside down, then slacken the nuts right to the end of the thread - this helps when refitting as the washer catches on the rear mech if it's not pushed right back. This is usually when I take the tyre off the rim as it's easier to do when on the bike.
Pull the wheel up and out of the dropouts**, it will still be attached by the electric cable but there is enough slack to seperate the wheel from the bike. Remove the old tyre and inner and replace with the new ones, refit is the reverse of removal. It's not hard but takes a bit of practice, you don't need to force anything except maybe the tyre bead over the rim.
If you forgot the direction arrows then don't worry as they aren't anywhere near as important as you'd think.
* Quite a rare size, Argos do a big roll of spanners for £15 that includes it if you need one.
** Watch how the sprocket goes on the chain as it looks very different with no tension on the mech.