I Want to upgrade my PSU and it is top mounted. Most PSUs I look at have an intake fan on the top. In my PC case, there is no vent in the top for it to intake the cool air. Is it okay to mount it upside down so the fan can still get air, even though it will suck the air out of the PC case?
Yes, it is okay to mount the psu "upside down" as you put it. The psu fan will act as an exhaust fan and help to remove warm air from the case.
BTW - There is a very good historical reason why power supplies are installed near the cpu. Intel kept manufacturing cpu's that kept runnning hotter and hotter. At the time there weren't any cpu heatsinks with fans as we know them today. In addition, most cases did not have much in the way of case fans. As a result Intel specified the psu should be installed near the cpu. Intel intended for the power supply fan to help keep the cpu cool. Of course a lot of that has changed over the years.