Hi, I want to create a function where users can mass update their existing data by uploading the CSV file. This is the code. But I get an error.

public class FileUpdate {
    public string nameFile{get;set;}
    public Blob contentFile{get;set;}
    String[] filelines = new String[]{};
    List<Account> accstoupload;

    /***This function reads the CSV file and inserts records into the Account object. ***/
    public Pagereference ReadFile()
    {
        try{
                //Convert the uploaded file which is in BLOB format into a string
                nameFile =blobToString( contentFile,'ISO-8859-1');
                
                //Now sepatate every row of the excel file
                filelines = nameFile.split('\n');
                
                //Iterate through every line and create a Account record for each row
                accstoupload = new List<Account>();
                for (Integer i=1;i<filelines.size();i++)
                {
                    String[] inputvalues = new String[]{};
                    inputvalues = filelines[i].split(',');
                    
                    Account a = new Account();
                    a.Name = inputvalues[0];
                    a.ShippingStreet = inputvalues[1];       
                    a.ShippingCity = inputvalues[2];
                    a.ShippingState = inputvalues[3];
                    a.ShippingPostalCode = inputvalues[4];
                    a.ShippingCountry = inputvalues[5];
        
                    accstoupload.update(a);
                }
         }
         catch(Exception e){
                 ApexPages.Message errormsg = new ApexPages.Message(ApexPages.severity.ERROR,'An error has occured reading the CSV file'+e.getMessage());
                ApexPages.addMessage(errormsg);
         }       
        //Finally, insert the collected records
        try{
            insert accstoupload;
        }
        catch (Exception e)
        {
            ApexPages.Message errormsg = new ApexPages.Message(ApexPages.severity.ERROR,'An error has occured inserting the records'+e.getMessage());
            ApexPages.addMessage(errormsg);
        }    
        return null;
    }
   
   /**** This function sends back to the visualforce page the list of account records that were inserted ****/ 
    public List<Account> getuploadedAccounts()
    {
        if (accstoupload!= NULL)
            if (accstoupload.size() > 0)
                return accstoupload;
            else
                return null;                    
        else
            return null;
    }  

    public static String blobToString(Blob input, String inCharset){
        String hex = EncodingUtil.convertToHex(input);
        System.assertEquals(0, hex.length() & 1);
        final Integer bytesCount = hex.length() >> 1;
        String[] bytes = new String[bytesCount];
        for(Integer i = 0; i < bytesCount; ++i)
            bytes[i] =  hex.mid(i << 1, 2);
        return EncodingUtil.urlDecode('%' + String.join(bytes, '%'), inCharset);
    }   
}

This is the error

Method does not exist or incorrect signature: void update(Account) from the type List<Account>

I get an error in this line:

accstoupload.update(a);

May I know how to debug this? 
If we want to have a mass upload record from CSV files to salesforce, we can use

accstoupload.add(a);

But why I cannot change from add to update when I want to update the existing records?