Are any of windows-7 users struggling to change your clock from 24hr to 12hr or vice versa? Are you trying to change it in Date and Time option? It’s not there, I will show you.
Open Region and language in Control panel.
Under Formats, Change Short Time and Long Time.
If you set that to Small lettered “tt hh:mm” in Short Time and “tt hh:mm:ss” or “tt h:mm:ss” in Long Time , the clock will set to 12hr Time Format.
If you set that to Capital lettered “HH:mm” or “H:mm” in Short Time and “ HH:mm:ss” or “H:mm:ss” in Long Time , the clock will set to 24hr Time Format.
Check this video for more.