Gotta take it all apart (including removing the drum) to be able to get the new belt on. Unfortunately, you did not give the brand name or model number, so I can't tell you how to proceed, but in most American models, you remove or lift up the control console, then the dryer top, then the front, and then the front bulkhead. You may not have to actually remove the drum, but you might be able to feed the new belt over the drum, but then you have to re-connect the idler wheel by feel, and if you've never done that before, then it is time to call a professional appliance repairman.
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Which brand and model of Dryer?
Gotta take it all apart (including removing the drum) to be able to get the new belt on. Unfortunately, you did not give the brand name or model number, so I can't tell you how to proceed, but in most American models, you remove or lift up the control console, then the dryer top, then the front, and then the front bulkhead. You may not have to actually remove the drum, but you might be able to feed the new belt over the drum, but then you have to re-connect the idler wheel by feel, and if you've never done that before, then it is time to call a professional appliance repairman.