Can I reduce/change the margins given in the template of a journal?
The entire point of the template is to make certain that everybody is on the same page (no pun intended) as to what the format is. Changing it defies this purpose and will almost certainly be noticed – if the journal uses this template for production at all.

While nobody except the journal staff knows how exactly they would react to what you intend to do, I would prophesy that at best they directly return the manuscript to you and ask to properly shorten your manuscript.