Q: How do I get the migration assistant disk to not be my startup?

I used migration assistant to move files from my 2008 macbook pro to my refurbished 2011 macbook pro. The older computer was running Sierra, while the newer computer came loaded with El Capitan. At the end of the migration process, I let migration assistant leave the files in a disk on my new laptop. Now the 2011 computer starts up with the old desktop and files from the old computer, and runs very slowly.

Should I load Sierra on the 2011 computer to stop this problem? The newer computer has software loaded that I can't access, and that's part of the reason I bought it.