Thread how to change sleep time

In my project I have function as defined below:

void start(std::chrono::milliseconds delay) override
{
    mDelay = delay;
    if(mThread == nullptr)
    {
        mThread = std::make_unique<Thread>([&]()
        {
            sleep(std::chrono::milliseconds(mDelay));
        });
    }
}

When I call the method for first time the thread is created and goes to sleep for mDelay period. But I need to change the mDelay time again but for a lesser time period that earlier one supplied.

But when I call it again I am unable to cancel the previous sleep and start a new one with defined mDelay?

How can I achieve the same?