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contracts...MyContract.sol

// SPDX-License-Identifier: UNLICENSED
pragma solidity 0.8.0 <0.9.0; 

contract MyContract{
    
   address owner;
   string public mobileNo;
   
   modifier onlyOwner(){
       require(msg.sender == owner);
       _;
   }
   
   constructor() {
       owner = msg.sender;
   }
   
   function updateMobileNo(string memory _mobileNo) public onlyOwner{
       mobileNo = _mobileNo;
   } 
   
}