I have an old dishwasher that was hooked up to the cold water instead of the hot . How do I switch it

Depends on what type pipe was used and is anything else in the house crossed and what kind of valves you have under sink and how handy you are with tools.

Doug is right. Plastic, white/tan PVC/CPVC, is DIY friendly. There are ways of dealing with the translucent PEX and copper if you don't have a crimper or a torch and the experience to use them. Post back telling us what your pipe looks like. Also suggest your skill level. You need to cap off the tee on the cold line and add one to the hot line.