you usually warm the oil by running the engine.
But as you do not know how old the oil is; or if it is contaminated with water or rust, I would be inclined to drain off the old, fill up with new, start it (if you can) to get it hot, then drain off again, then refill with new. As it is such a small engine it will not take much oil and it does not need high-tech modern synthetic oils, so the cost will be very slight.
It is good practice to do an oil change before putting it away for the winter (or see Dan's website that I posted before).
But as you do not know how old the oil is; or if it is contaminated with water or rust, I would be inclined to drain off the old, fill up with new, start it (if you can) to get it hot, then drain off again, then refill with new. As it is such a small engine it will not take much oil and it does not need high-tech modern synthetic oils, so the cost will be very slight.
It is good practice to do an oil change before putting it away for the winter (or see Dan's website that I posted before).