My advice where you merely want to replace fluid rather than do any work involving disconnection is to make sure that you don't actually need to "bleed" as such.
Connect a clear pipe to one bleed nipple at a time, dipped into a jar of fluid, loosen the nipple half a turn, and then gently depress the pedal forcing fluid into the jar. Watch the colour until the new fluid comes through, then tighten and move on to the next.
Warnings:
NEVER press the pedal down further than you do in ordinary hard braking. The number of people reporting new master cylinder problems arising after bleeding the brakes is legion. Sliding the non-return seal in the MC over an unused part of the bore will damage it and cause new internal leakage.
Take care never to run out of fluid, or all your efforts will be wasted.
If a bleed nipple will not turn, DO NOT force it, or it will snap and cause even more problems.
Clean around the threads
Run a few drops of brake fluid around the threads
Leave it overnight
Try again very gently
Put a well-fitting ring spanner round the nipple and apply quite a lot of force. It's a matter of difficult judgement how much (!)
While exerting the force, hit the nipple end-on with a hammer. NOT sideways.
When you get any movement at all, run more fluid round, and work the spanner to and fro a tiny bit, not forcing, but gradually increasing the movement until it seems free. Then proceed with the sequence.
Good luck. You ought to produce a system that has been entirely flushed out and replaced with new fluid, without any real bleeding (and risks) involved at all.
