Need to change page size from Legal to Letter in Acrobat DC (non Pro)

Do not change the page size in Acrobat. Reconvert from word using the correct size. 

If you are actually using Acrobat DC, and NOT Reader, yes you can crop the pages very easily.

Tools > Print Production > Set Page Boxes.... set crop values for top and bottom that suit your situation. e.g. If your content is centered on the Legal size, use 1.5" top and 1.5" bottom. or If your content is at the top and you want to crop off the bottom, just set bottom crop to 3". Apply to All Pages and you're done.

Sorry, even if you had Acrobat Pro,  it's not realistic. (BTW, I'm not sure from what you write if you are using Acrobat Reader or Acrobat Standard.)

At this point, if the document is not too elaborately formatted, your best bet is to go to the original document, reformat it, and resave it into a PDF. The 2nd best plane is to export it into Word, do the reformatting, and resave it as a PDF. 

Doing this in Acrobat is not viable because Acrobat is neither a page layout application or a document program.