I changed the pipe connecting the hot water line to the water heater and now certain areas of the house have decreased water pressure. Specifically, the kitchen sink and the hot water knob in the downstairs bathroom. The upstairs faucets have all been fine. What can I do to restore pressure to those areas? Do I need a shorter pipe? (I used a 12 inch and it's slightly crimped, that's my only guess right now) Thanks

First thing to do is remove and clean or replace aerators on the faucets affected. Sounds like your plumbing work created some sediment that is now restricting those faucets. If your pipe repair included soldering, you probably used too much solder.

Likely the pressure is the same but the volume has decreased. Replace that pipe with one equal to or larger diameter than the original, throw out the crimped pipe.