there are 4 holes in the front of the cd head unit in the dash. if you want to do it the make shift way you can get 4 slim electrical screwdrivers and push 1 into each hole until there is a slight resistance then pull the whole unit out.
You can also buy the actual 'keys' to do the job off e-bay for bout £3. i have a set and they fit all french cars and certain others too. very useful. The hole left by the factory fit unit is standard DIN slot so any aftermarket stereo will fit it. You will find that your steering remote control becomes obsolete if you put a new stereo in. also that typical standard speakers in 307's are only 15w and you may blow em up with a new stereo. Invest in some higher wattage speakers if you are upgrading or else you wont get the benefit of buying your new stereo.