The solution below from the SUM guide worked great in phase TABIM_UPG. No need to stop the update.
cd <update directory>\abap\bin
SAPup set procpar gt=scroll
Set the parameters you want. Check SAPUpConsole.log and the next import group to run will use the new parameters:
SAPUpConsole.log
Before: -Dmainimp_proc=2 -Dparallel=3
After: -Dmainimp_proc=2 -Dparallel=6