I have a computer power supply (PSU), input voltage is 120-240V, I assume I can safely plug it into a 240V outlet. I have one those outlets in my garage, problem is that it is 6-20R type receptable and the PSU plug is 5-15P and does not fit. What should I do while being code-compliant? Change the 6-20R receptacle to a 5-15R and label it with 220V? Or, is there something like a 6-20R to 5-15R adapter that could be used? Please advise.
Assuming that your computer power supply accepts its power through a C13 socket, you could use a pigtail to make the connection.
Example: 6-20P to C13 Power Cords
Cables like this are commonly used in data centers to get 240V tolerant equipment attached to the rack PDU.