My PC is running 5 work units simultaneously. I'd rather have do one start to finish then move on. How do I make those changes? Or it it doing that because of the multiple processors in the computer? Either way, I'd think one at a time would be best. crank it out as fast as possible then do the next.

BOINC will launch one task per CPU/core in your computer, and an additional one if you have a capable GPU. Switching it to 1 task at a time will not speed up the processing of that one task, nor will it spread that one task across all cores/CPUs.

But if you still want to change it, go to your preferences, edit the preference venue the computer is in (if you only have the default preference, then this will be the one to edit), and modify the line that reads:

"On multiprocessors, use at most
Enforced by version 6.1+"

It is a total percentage of CPUs. So for your quad core, 25% would be one CPU. Save the changes, then open BOINC Manager on your local machine, select the Projects tab, highlight SETI@home, then click on the Update button to download the newest preferences.