One partner changes, the pair continues
On Players, open the pair and replace the one partner in place. The pair keeps its identity — its block position, seed, played games and schedule — with the new name in the outgoing player's seat. This is the mid-event tool: an injury on day two doesn't unwind the draw.
Both partners, or a re-pair before play
Before any games are played you can simply dissolve and re-pair: return the players to the pool and pair again (tap two, or Auto high-low). Once results exist, prefer the in-place replacement — re-pairing makes the played games belong to a pair that no longer exists.
Afterwards
If the replacement changes the pair's combined strength materially, the seeding-stale flags behave as for singles — see Fix "snake-seed may be stale".