So you're using Visual Studio to create the report and when you go into Preview you get the error I assume. To be clear here it sounds like you're trying to build and deploy your report, not do a test run of it. Deploying the report will cause it to try to connect to the target server and deploy the report to it. You only need to do that when you've got the report working. To change the SSRS instance that the report will be deployed to you should right click the project in Solution Explorer, then select Properties. In that UI there is the TargetServerURL which specifies what SSRS instance you want to talk to. Point it to the SSRS instance you want and then try deploying again.
If you just want to test your report without deploying then you can do that via the Preview tab in the designer. In this case it'll use the Data Source(s) you've configured to pull the data. This can come from any databases (or files or whatever) and has nothing to do with the target server.