Class----------->
public class InvestmentSettingAura {
    
    @auraEnabled
    public static Investment_Settings__c getInvSetting(){
        
        List<Investment_Settings__c> InvSettingList = [SELECT Id,AutoBid__c,Investor__c,Max_AutoBid_Percentage__c,Maximum_exposure_to_1_Seller__c,
                                                       Maximum_Auto_Bid_Amount__c,Maximum_exposure_to_1_Customer__c,
                                                       Maximum_Credit_Grade_for_AutoBids__c
                                                       FROM Investment_Settings__c
                                                       WHERE Investor__c =:UserInfo.getUserId()];
        
        if(InvSettingList.size()>0){
            Investment_Settings__c invSet =new Investment_Settings__c();
            invSet = InvSettingList.get(0);
            return invSet;
        }
        else
            return null;
    }    
    
    @AuraEnabled 
    public static List<String>  autoBidGrade(){
        List<String> options = new List<String>();
        Schema.DescribeFieldResult fieldResult = Investment_Settings__c.Maximum_Credit_Grade_for_AutoBids__c.getDescribe();
        for (Schema.PicklistEntry f : fieldResult.getPicklistValues())
            options.add(f.getLabel());
        return options;
    }
    
    @AuraEnabled 
    public static string  saveInvestmentSettings(String invObj){
        Investment_Settings__c invSet =new Investment_Settings__c();
        invSet = (Investment_Settings__c)JSON.deserialize(invObj, Investment_Settings__c.class);
        //invSet = invObj;
        SavePoint sp = Database.setSavepoint();           //[is covering in test class]
        
        try{
            update invSet;
            return 'Success';
        }
        catch(Exception e){
            Database.rollback(sp);
            return 'Error : '+ e.getMessage();
        }
    }
}

Test Class ------------>

@isTest
public class InvestmentSettingAuraTest {  
     public static testMethod void testInvestmentSetting1(){
          Profile p = [SELECT Id FROM Profile WHERE Name='System Administrator']; 
        user u = [select id from user where name='TMC Admin'];
         
        Investment_Settings__c invset = new Investment_Settings__c();
        invset.CurrencyIsoCode = 'SGD';
        invset.AutoBid__c = true;
        invset.Investor__c = u.id;
        insert invset;
        Investment_Settings__c invset1 = new Investment_Settings__c();
         invset1.Id= invset.Id;
        invset.CurrencyIsoCode = 'SGD';
        invset.AutoBid__c = true;
        invset.Investor__c = u.id;
        update invset;
        
        InvestmentSettingAura.getInvSetting();
        InvestmentSettingAura.autoBidGrade();
        InvestmentSettingAura.saveInvestmentSettings('invObj');
    }
}