Switch my linked device to become main account

Maybe when you acquired a new device you added it as a Whatsapp linked account. A linked account cannot add new contacts so maybe you want to convert the new device to be the main account.
On the linked device click in the top right and select log out
When you open WhatsApp again, after selecting language and agreeing to T&Cs, enter your phone number
because your new device has the same phone number as your account, it’ll see that you’re an existing user and so will now connect to your old account.
It will present a QR code and ask you to scan with your old device to connect it, you will need to allow the app access to various resources then it will transfer all data to the new main device
The old device will now have been logged out as it’s no longer your primary device you may now want to add that as a linked device..