How do I switch from my main line to my second line on my iPhone 14?

Except for Cellular data usage, you don't switch. You can choose which line you use when you place calls or send messages.

click here ➜ Using Dual SIM with an eSIM - Apple Support

For Cellular Data:

1. Go to Settings.
2. Tap either Cellular or Mobile Data.
3. Tap Cellular Data. 
4. Tap the number you want to use for cellular data. 

The feature on an iPhone is as follows based on the way you are trying to call.

Watch the GIF below to change the Voice line. This has to be done each time you receive a call and want to call back from another line other than through which it was received.

Alternatively, you may change the Voice line to be used under Contacts. And dial using contacts and not using the Recent call list.

While making a fresh call using dial pad.