Andrew Gabriel wrote:

Redoing a joint like this which was done OK originally is not difficult.
You don't have to clean the pipe end up, just make sure the pipe end is
tinned all the way round (i.e. that it was a good joint originally).
You might not be able to push the coupler over the solder. In this case,
you will have to heat the pipe end in order to push the cleaned/fluxed
coupler on, [...]

When you get the old joint apart, wipe the end of the pipe with a
slightly damp cloth while the solder is still molten (use a swift
motion, to avoid cooling things down). This will remove surplus solder
and leave a nice clean tinned end for the new joint. It's similar to
cleaning the tip of a soldering iron on a damp sponge.

--
Andy