the standard way is to go to options> security settings > general > blackberry menu > wipe device. this will completely wipe it and bring it back to "factory new". after that, you'll want to redoanload your service books just in case.
if that doesnt work, wipe the device using Jl_cmder. it's the best way to completely clean it, then upload your handheld OS. you can get further guidance from the faq here: How do I wipe the BlackBerry using Jl Cmder? - BlackBerryFAQ
just make sure you have desktop manager installed at a copy of your handheld OS.
as for picking up other towers, it should auto select those towers only if there's no signal from your primary service provider.