There are no hard and fast rules for what to do with the change. Generally try to avoid the change and use the Max button extensively at sending. The most problematic type of change is what has anonymity set 1 (red shield.) You should treat it as a kind of toxic waste (handled with great care).



Warning You want to avoid merging anonymity set 1 coins with anonymity set > 1 coins wherever possible, because this will link your anonymity set > 1 coin to the coin you merge it with. Note that, this is also true if you merge them in a mix, however that is slightly less problematic, because some blockchain analysis techniques become computationally infeasible.



It is also important that you do not send different coins to the same receiving address (even if performed as separate transactions) as this will also link the coins together, damaging your privacy.



Your Options:

- If you do not care about linking the history of the coins because they are all from the same source then you could combine them in a mix (queue all the change from the same source until you reach the minimum input required to mix, currently ~ 0.1 BTC).

- Mix with Joinmarket.

- Donate them (e.g. to the EFF)

- Spend them on something that is not a particular privacy risk (eg. gift cards).

- Open a lightning channel.

The ultimate solution is to 'close the loop' i.e. spend a change coin without merging it with other coins, do not generate it in the first place by sending whole coins.



Further reading: Chaumian Coinjoin