If I have mutible versions installed How can I run dfx start with a specific version ?
You can edit the “dfx” property in your project’s dfx.json file.
You can set it to any version cached on your machine, run
dfx cache list to see the versions you have installed.
I think you have to specify the version locally even if you specify the version in dfx.json on the project side, but does it work if you just change dfx.json?