Pull the cranks, remove the non-drive side lockring and adjustable cup as oddjob2 recommended and then pull out the spindle and bearings. Don't remove the fixed cup yet as you can clean and work on it through the bb shell.

If the spindle's and cups' bearing tracks are smooth, just clean everything and replace the bearing balls. Loose balls can be used in place of retainer held balls but you will need a couple of extras per side. "Glue" the balls in place with a bead of grease to hold them in position as you place them in the cups. Add the spindle, thread in the adjustable cup, add the lockring, adjust for minimal to no play and tighten the lockring.

If the spindle or cups are damaged, the easiest replacement is a cartridge. Removing the fixed cup requires a specific tool or it can be clamped in a bench vise and the frame used as the turning lever. Remember the fixed cup is left-hand threaded.