Here is the first pattern to upgrade a smart contract via a middle registry contract. Which returns the new version contract. Below is the structure of Registry Upgradeable Smart Contract Pattern.

Onwer.sol

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

contract ownerContract{
    
    address owner;
    event OWNERSHIPCHANGE(address indexed previousOwner,address indexed NewOwner);
    
    constructor(){
        owner = msg.sender;
        emit OWNERSHIPCHANGE(address(0),owner);
    }
    
    modifier onlyOWNER(){
        require(msg.sender == owner,"only Owner Allowed");
        _;
    }
}

registry.sol this contract act as a middle man to get the address of upgraded of latest backend contract.

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

import './owner.sol';

contract registry is ownerContract{
    
    uint latestVersion = 0;
    
    mapping(uint=>address) versionHistory; 
    
    constructor(){
        
    }
    
    function updateVersion(address newUpdatedContract)public onlyOWNER{
        latestVersion++;
        versionHistory[latestVersion] = newUpdatedContract;
    }
    
    function getAddressofLatestContract()public view returns(address){
        return versionHistory[latestVersion];
    }
    
    function getContractByVersionNumber(uint version)public view returns(address){
        return versionHistory[version];
    }
}

BackendContract.sol this is the main contract to perform functionality.

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

contract ABC{
    
    uint z;
    
    constructor(){}
    
    function add(uint x, uint y)public {
        z = x + y;
    }
    
    function getValue()public view returns(uint){
        return z;
    }
}

Limitations:

• Storage issue.