Seriously, I had this exact problem a week ago. Called the plumber. There are a lot of DIY things I'll do, but plumbing (and electrical) are where I'm calling a pro.

Of course, the plumber couldn't make it for almost a week. So I looked into it on YouTube. One trip to Home Depot and about $20 later, it took me 20 minutes to replace.

It's seriously just:

• Buy the same flush valve you have now
• Turn off water
• Drain tank (needed to use sponges to get the last of it after flushing)
• Disconnect water line
• Disconnect the hose from the fill valve to the flush valve
• Unscrew plastic nut holding the fill valve in at the bottom of the tank
• Remove the hold fill valve
• Install the same restrictor on the new flush valve as is on the old one (they were color-coded and came with the new one)
• Adjust the new fill valve to the height of the old one (it's a screw adjustment on the valve)
• Install the new fill valve by reversing the steps above - screwing in the new nut and connecting the water line
• Turn the water back on
• Allow to fill and then flush
• If the water level in the bowl is too high or low, turn the screw on the top of the fill valve to adjust it.
• You're done.

That many steps makes it seem more complicated than it is. It's actually really easy.