Changed the alternator belt yesterday and have a couple of tips to make it easier. You can do it from above which saves removing the undertray but normal length spanners are too short and awkwardly placed to get enough purchase on the 13mm nut of the belt tensioner. This has to be turned to give enough free play in the belt so that it can be removed from the pulleys.
I used a 14 inch breaker bar with a 13mm socket attached to the belt tensioner nut, with the bar handle projecting forward, horizontally under the engine mounting. This way there is plenty of room to get a good downward force on the bar handle which easily releases the tensioner. When fitting the new belt try to straighten out the kinks first as much as possible and fit it over the bottom pulleys and the tensioner wheel first. Then, with the tensioner loosened, fit it over the topmost pulley
I used a 14 inch breaker bar with a 13mm socket attached to the belt tensioner nut, with the bar handle projecting forward, horizontally under the engine mounting. This way there is plenty of room to get a good downward force on the bar handle which easily releases the tensioner. When fitting the new belt try to straighten out the kinks first as much as possible and fit it over the bottom pulleys and the tensioner wheel first. Then, with the tensioner loosened, fit it over the topmost pulley
