Depends how old it is. Is it low level with a flushpipe, or high level with a flushpipe or does the cistern actually sit on the back of the toilet pan?
depends if it is a close coupled, low level or high level cistern, what pipework runs in and out of it, lead? copper? cast iron? plastic? how old the house is?
would need more information, to give a more informative answer, also what plumbing skills you posess?