Hi! About a year ago, I got a new water heater installed. Here's the question - I had terrific flow from my bathtub upstairs but after the installation, the flow is so slow. I had the installer look at it but he said it was the pipes. Honestly, he broke my handle in the bathtub and didn't want to do more. What could be the problem?

And you waited a whole year to ask us for help?
I had the installer look at it but he said it was the pipes. Honestly, he broke my handle in the bathtub and didn't want to do more.
Send your installer back to Plumbing School 101. He really needs a refresher course.
First off do you flush the sediment out of your heater on a regular basis?
What I think has happened is that when the heater was installed some built up minerals on the pip[e walls were disturbed and are not blocking the inlet cartridges on the faucets affected.
The solution would be to shut the water off to the affected faucets and open them up. Remove each cartridge and check the inlet ports for trash. On each faucet, before you reassemble, place a pan over the open valve body and turn the water back on to flush out the supplies and valve. Do this on all the faucets that have low flow and don't forget to unscrew the aerator from the spouts and check and clean the screens. Good luck, Tom