Hold down the volume up and volume down buttons simultaneously for 3 seconds. The LED will start blinking rapidly, indicating it's in pairing mode again.
HOW TO switch between the two devices:
This doesn't seem to be like Bluetooth Multipoint — I don't think the headset automatically switches between two sound sources, it seems like you have to manually switch them.
The quick start guide does say that the device supports Bluetooth V5.0, and multipoint connectivity began being supported in Bluetooth 4.0, so there's a chance these headphones can automatically switch?
This page suggests stopping audio playback on one device and starting it on another. But that didn't work for me.
The manual says "Each time you power your Soundcore Life U2, it will automatically connect to your last successfully connected device if available"
That seems to work for me.
You can go into your phone/PC's settings / control panel > Bluetooth > Anker U2 > manually Disconnect (NOT "Forget" or "Remove Device"!). Then, on the other device, do the same with U2's Connect button.
This seems awfully clumsy, but it DOES work for me.