No root required*
This is pretty simple, go to the dialer and enter;
*#*#4636#*#*
Click on Phone Information
Scroll down to Set preferred network type
Select that one that you want
hit back
My phone kept switching to edge because the wcdma signal in my area is very low but 3 bars of edge is worse than 1 bar of hsdpa+ (no lte in enterprise al) so I choose wcdma only and I always have signal. You can set preferences or choose certain signals only but be aware if you change to a specific signal only and it's not there you will loose service... If you ever need to reset the settings and can't remember the code removing the sim card and reinstalling it resets back to default.
Here's where I found this I take no credit but I can confirm it works on the at&t version of the One.
http://forum.xda-developers.com/showthread.php?t=1636345
This is pretty simple, go to the dialer and enter;
*#*#4636#*#*
Click on Phone Information
Scroll down to Set preferred network type
Select that one that you want
hit back
My phone kept switching to edge because the wcdma signal in my area is very low but 3 bars of edge is worse than 1 bar of hsdpa+ (no lte in enterprise al) so I choose wcdma only and I always have signal. You can set preferences or choose certain signals only but be aware if you change to a specific signal only and it's not there you will loose service... If you ever need to reset the settings and can't remember the code removing the sim card and reinstalling it resets back to default.
Here's where I found this I take no credit but I can confirm it works on the at&t version of the One.
http://forum.xda-developers.com/showthread.php?t=1636345