At the BASIC prompt (which from memory looked like either ] or >
type PR#6 Enter to boot from 5.25" internal drive, or PR#5 to boot from external 3.5" Unidisk (if it has the updated MON ROM, the original ROM in earlier //c shipped did not have the intelligence to recognise and use 3.5" diskette drives).All this is from memory, without checking, I could be wrong.
If you do not see the BASIC prompt, press reset (if it has a reset key) as then I think the BASIC prompt should appear. Another possibility might be Control-C or Control-C Enter