Most people do not realise there are two.
The account number
The one you sign in with, verified once with a code. It is your account — sign-in finds the account that owns a verified number.
To change it, do it from your settings. A code goes to the new number, which is how the account knows it is really yours.
Do it while you still have the old number if you can. That is the easy version.
The delivery number
The number a courier rings when a parcel is out for delivery. Not the same thing, and changing it does not touch your account.
It is there so you can give a different contact for a particular delivery — a workplace, a relative, whoever will actually be there. Setting it never affects how you sign in, which is deliberate: a delivery contact that could claim your sign-in number would leave your account reachable at a number nobody verified.
If you have already lost the old number
The account is not lost — it lives on Metzee's servers, not on the phone. But the number is how it proves it is yours, so the route back is harder.
Get the SIM back first, if you can. A replacement SIM on the same number restores normal access and is the shortest path by a distance.
If the number is gone for good, contact Metzee under Account & login. Expect to be asked things only you would know — recent orders, what you bought, the bank account on file if you sell. That friction is the protection working. I lost the phone with my Metzee account
What not to do
Do not make a second account. One verified number belongs to one account, and a new one splits your orders, your ratings and your history. Can I have two accounts?
Do not give your code to anybody, including anyone offering to help you recover the account. A code is for you to type. Messages that are not from Metzee
If you sell
Update it before you need it. A payout failure or an order problem is reported in the app and waits for you — but a number you cannot receive on makes every other route slower. Payout failed
The thing worth doing today
Check which number your account uses. People genuinely do not know, and the moment you find out should not be the moment you need it.
More: how to keep your account safe, why Metzee asks for your phone number, I lost the phone with my Metzee account.