Sometimes it is just the idler pulley bearing that screeches - not the belt, a loose belt is more likely to screech when alternator loaded up ( try turning all electrics on and 'blipping' the throttle to provoke screech ).
Have replaced aux belts on Jazz MK1 ( where you need to slacken off alternator pivot bolt and another bolt that allows alternator to be pivoted to tension the belt - neither are easy to get to - you need some decent tools ) and Civic which has spring loaded tension - as does the MK2 onwards )
If you are going to replace belt make sure you have a diagram of where it runs, when I changed the one on Civic ( when I replaced aircon clutch I thought - mainly because getting to the belt was a big PITA - i may as well replace belt as well, I got the belt from Cox motors ( was not gonna fit a cheapo belt after all the trouble to access it - and the belt came with a diagram of how the belt is threaded over pulleys - showing the right and wrong ways, I would have been stumped without it - and it is easy to get in a muddle - you cannot really photograph it ) the belts have no fixed replacement mileage or time, belts are only replaced when cracked or fraying and can easily do 150,000 miles ( polyurethane with kevlar cords ).
Take wheel and liner out of the way as a lot of belt is hiding behind a steel chassis member, you need to get to bottom run of belt anyway as that is where you measure the tension.
I have attached PDF of Haynes manual pages for aux drive belt - The figures for deflection given by Haynes are
new belt - 4.5 to 6mm ( the belt will stretch slightly as it beds in )
used belt - 7.5 to 10mm