If you have paired the headphones or speakers with multiple devices, they should automatically switch the Bluetooth connection to the last connected device.
However, if the headphones or speakers have previously connected to a number of other devices, just turning on a different device may not always allow a connection. If this is the case, use the Bluetooth setting on the device to connect with the headphones or speakers.
Hint: If a connection still can't be established, check the Bluetooth setting on other paired devices to find the device currently connected with the headphones or speakers, and disconnect that Bluetooth connection. After that, use the Bluetooth setting on the desired device again to connect with the headphones or speakers.
If the issue is still not resolved, refer to the article The headphones cannot pair with the device, or cannot connect with the device once paired.