I have a hot water baseboard that needs replacing(the whole thing).It is a 6 foot section.
What are the steps that have to be done in order to do so. I only can see one shutoff valve before the section. This is down in the basement so I am about 15-20 feet away from the gas boiler.The only shutoff is at the boiler.After I am done do I need to bleed the sysytem?If so how?. Help!!

Turn off power,drain some water out of boiler to prevent a mess upstairs.

Cut out old section and replace with new. While you are at it you can install a special 90 degree elbow that has a small pipe thread area at the top of the elbow. To that opening you can screw in a cheap key vent or coin vent bleeder. Repipe new unit to fit and solder in. Then attach the small pipe fitting (bleeder) I mentioned before.

Refill system and bleed at the highest area first then check all other bleeders if your system has them. Then go back and rebleed the highest area again. Sometimes air can be a bit difficult to remove and after the unit heats up you may have to bleed a few more time to be sure all air is out of the system. While you are doing all of this make sure you check your work (solder joints) for leaks.

Recommission (start up boiler) and run test.

If your system has not been serviced in awhile you might also consider isolating the expansion tank and draining it. Then open the water valve to the expansion tank.
If you have a bladder style tank do not try to drain that style just leave well enough alone.