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If everything in the upstairs bath has poor cold water pressure the most likely culprit is a broken shut off valve to the upstairs. You could have the same problem with the hot water. The shut off to the tank could be broken. Either or both could be due to a loose washer in the valve
Where is this valve and is this something you would recommend for a home owner to do or should I hire a professional? If I hire a professional, how long would this take to fix and any idea the price range?
The valve for the water heater would be right above the heater on the cold water line feeding the tank. No telling from here where a valve for the second floor might be. If you are real handy it might be something you could tackle but very often when the washer breaks loose it's because the screw holding it in place has broken. This mean you need to drill out the old screw and tap new threads for a replacement screw. Maybe not a DIY project.
How long it would take would be at best an educated guess from here. Cost for a plumber could range from $150 to $200 possibly more