Why wont my phone let me change my apn settimgs

[AC Question]

I open the APN menu and press the ± key try to type in APN settings but it exits me back out immediately......It won't let me not even type in one word

 

[B. Diddy]

Welcome to Android Central! Which phone? Verizon/Sprint phones often won't let you change the APN (or make it difficult).