You won't stretch the belt - you'll pull the staple out . . .
Like Joe said, you have to "derail" the belt (take it off the large wheel) to lower the machine. That is one advantage many other machines have over the Singer - you don't need to take the belt off to put them to bed.
I've seen a lot of Singer treadle irons with the derailer missing - so if you don't know what we're talking about, that could be the issue.