They just snap over the housing. Just gently stick a screwdriver between the cover and the housing and twist it it will come off easily. Do that to all three sides starting with the top.
If you are using the Mach460 amps, you are pretty much stuck using the stock tweeters since the crossovers and amps are meant for their inductance and impedence. If you have an aftermarket system, you can use a different tweeter, but make sure you use the right crossover. Just rememver you don't want to mix passive and active crossovers if you don't have to.
If you get the aftermarket tweeter, you could always use the stock Mach360 tweeter housing.