Front speakers take about an hour to replace, only problem for me was the connector was difffernet, so I had to cut them off and pop on some different ones. Using a speaker adaptor on the fronts lets you fit 16cm speakers instead of the standard cars 13cm.... very well worth it, transforms the noise.
Unless your a bit brave, leave the rears to a professional. I did it myself but i'm not easily scared off a project.Takes about 3 hours first time out. Bound to break at least one of the little plastic poppers that holds the plastic nterior panels on.
Look in the DIY section for guides on how to do all this.
MikeyThemini also does little speaker plug adaptors so that you dont hvae to cut the original wires off and in the future can easily replace the oem ones if required. Definitely get some 16.5 cm pods from him though as this really opens up options for the larger, better components.