Its not obvious which one you mean. There are 3 things to change and you should really change all three as if your aux belt flys off it can take out the cam belt. (1) The top fixed idler wheel (2) the spring loaded (and expensive) 'tensioner' and (3) the eccentric pulley which is the actual adjustment for tensioning the belt against the spring loaded tensioner.
The only easy one to change is (1). Changing (2) is not too bad but access is tight. Changing (3) needs the engine mounts on that side slackening and the engine dropping. Thus, its quite a tricky job on the 20V and I would rate as 'very experienced DIY' job. Parts from ebspares or other Alfa supplier.