Green’s Dictionary of Slang

(US) to reduce a committment, an activity; to lower one’s emotions.

1995		Lancaster New Era (PA) 19 Jan. 4/5: Slater [...] is too young the play this role, and not confident to dial down a little.
2000		Sentinel (Carlisle, PA) 10 Feb. D4/1: Later, they cool off, dial down and vow to think before they speak, but then the red demon rises again in fury.
2019		D. Winslow Border [ebook] ‘You shouldn’t bet. On nothin’. That’s how you got in this mess the first place. How you doin’ with that?’ ‘I’ve dialed it down’.