How do I hide my wifi password

Surely you had to tell her the password in the first place? There's no way to hide the password.

If you didn't tell her the password and you typed it in on her phone because it's the same password as other devices that you don't want her to have access to then I suggest you change ALL your passwords so they are unique.

If your concern is that she could give the password to her friends than KiltedTim's suggestion is best. There are things you can do on your wireless router. Google is probably the best help rather than me trying type a long list of instructions - and it's router dependent. It's called MAC address filtering - This means you tell the router the MAC addresses of the devices that can connect. You'll have to turn off the iPhone's private MAC address to do this and every time you get a new device or guest user you'll have to update the MAC filter list. If a new device tries to connect, even if they know the password, the router won't let them. The downside is that every time you get a new device or allow guests access then you have to find out the MAC address of their device and add it to the MAC filter list.