I assume you refer to the internal connectors between pipework and fittings.
These generally have a collet or ring at the pipe end that you squeeze towards the fittings to release their internal clamping ring; the pipe then pulls outwards but this can still need some slight pressure (force) to do this.
This image gives you an idea and illustrates the collet:
Refitting is simply a case of pushing the prepared pipe end into the fitting as far as it goes to secure. Ensure the pipe end is cut square and free of rough cut edges, any burr etc., will effect the integrity of the seal.
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