I was wondering this myself. I only was able to change the always on clock but the lock screen is always the same .
I went with my Mom who is in her 60s to make sure she didn't get ripped off by the salesman, and the way he did it the way it appears in magazines and in commercials with the two digits for the hour and the 2 for the minute on top of one another was for the only way for him to apply that setting and have it come up when you press the power key or volume rocker keys was to set the phones FONT settings to HUGE.
Which is a bummer cause HUGE just isn't feasible for the way and for the company I use and need my phone for. Having the FONT on HUGE shouldn't be the only way but unless they roll out a patch, which I don't think they will over a manner so miniscule, its best to wait for a stable ad free app that isn't a memory hog or malware or anything like that mess that comes with apps that promise the world and kill your phone!!!
Hope this helps
BTW This was at an AT&T Store in August of 2017