Warning: The first publication of the story in that ultimate anthology (Final Stage: The Ultimate Science Fiction Anthology) was “cut to shreds” by a ham-handed editor at Charterhouse, so your best bet is to read it in one of Silverberg’s later collections.
Also in need of some help is the model of time travel in the story, which is a mishmash of popular representations that no person at age eleven or elsewhen should be exposed to. Specifically, I would have enjoyed an attempt to square the Branching Timeline implied by the hole in floor with the single nonbranching, static timeline and Ex Nihilo paradox hinted at by the time-travel device. I truly liked that ex nihilo paradox, and wish it had been explicitly dealt with rather than swept under the carpet.
Ramazan: Assuming no one else had gone back and changed things before he did.