logical operators in if statements c++

The current code at the bottom works but I can't combine the if statements without shifting the ascii values to a position that I don't want them to be. The encryption is supposed to be only alphabet values. Capital and lowercase z's are supposed to loop around to an A. I have a teacher but she doesn't know so I would be grateful for any help. Thanks <3

This doesn't work...

if (sentence[i] == 'z' || 'Z')
{
sentence[i] = sentence[i] - 26;
}

And this doesn't work

if (sentence[i] == 'z' || sentence[i] == 'Z')
{
sentence[i] = sentence[i] - 26;
}

This works.

if (sentence[i] == 'z')
{
sentence[i] = sentence[i] - 26;
}
if (sentence[i] == 'Z')
{
sentence[i] = sentence[i] - 26;
}

Full code.

#include <iostream>
#include <string>

using namespace std;

class EncryptionClass
{
    string sentence;

public:

    //constructors
    EncryptionClass(string sentence)
        {setString(sentence);}
    EncryptionClass()
        {sentence = "";}

    //get and set
    string getString()
        {return sentence;}
    void setString(string sentence)
        {this-> sentence = sentence;}

    //encrypt
    void encryptString()
    {
        for(int i = 0; i < sentence.length(); i++)
        {
            if (isalpha(sentence[i]))
            {

                if (sentence[i] == 'z')
                {
                    sentence[i] = sentence[i] - 26;
                }
                if (sentence[i] == 'Z')
                {
                    sentence[i] = sentence[i] - 26;
                }

                sentence[i] = sentence[i] + 1;
            }
        }
    }
};

int main()
{
    string sentence;
    cout << "Enter a sentence to be encrypted. ";
    getline(cin, sentence);
    cout << endl;

    EncryptionClass sentence1(sentence);
    cout << "Unencrypted sentence." << endl;
    cout << sentence1.getString() << endl << endl;
    sentence1.encryptString();
    cout << "Encrypted sentence." << endl;
    cout << sentence1.getString() << endl;


    cin.get();
    return 0;
}