Turn power off to the boiler via fused spur. Get hose pipe or bucket open drain valve to release water in heating system. If using hose pipe its worth using a jublee clip to secure it to drain valve. Open air vents on rads in the room above pipes, might be worth opening all rad vents on 1st floor so the air will force all the water out of the pipes/rads. Remember to close all rad vents after draining system. Now you can cut pipes and re route etc. After thats complete make sure you have solder all joints and all rad vents are closed. Now you will have to refill system the system via the filling loop should be under boiler and looks like a braided pipe with a service valve or thumb handle to open and let cold water back into heating system. Depends how new your boiler is it maybe located in the boiler its self and might require a key to operate. This is still with boiler power off. Also you should open this valve slowly it takes a little longer to fill the system but i think its safer so not to disturb any sludge in the boiler. The boiler should have a pressure gauge on the front fill the system to roughly 1.5bar and turn off filling loop. Go around and bleed all air from rads on 1st floor. You will need to top up the pressure as you bleed the rads. repeat this until all rads are bleed. Then do a final top up so the pressure is 1.2bar. Check that the automatic air vent in the boiler is open this is usually at the top of the boiler or on top the pump. It will have a dust cap on it and should be open 1 turn will be fine if closed this will get rid of any trapped air in boiler.Turn boiler on and fire up. This is pretty much the basics this is at your own risk.