|This page in a nutshell: Renames are now done globally; please request the name change at m:SRUC.|
Requests about changing usernames are now handled centrally at meta.wikimedia.org:Steward requests/Username changes. The instructions here are not necessarily updated. meta:Global rename policy may better reflect the current situation.
Before user renaming was made global, i.e. simultaneous for all of the Wikimedia Foundation projects, including Wikipedia and Wikimedia Commons, this was the page where users could be renamed. Now user names can no longer be changed on individual projects such as Wikivoyage.
If the same user name is in use on more than one project, they have to be unified ("SUL") or else all but one will have to be renamed. If you are in this situation, you should probably very soon take steps to get a unique unified username. This involves showing the username is yours on all projects, if need be after either "usurping" the username on some projects or changing your username to something unique.
For Wikivoyage, there is the additional question about accounts used before 2013, i.e. before Wikimedia servers were used. These edits were imported with usernames prefixed (WT-en) or (WV-en). If you have such an account and want to merge your old editing history with your contributions here, local bureaucrats are no longer able to do so; inquire on meta:
It is not possible to delete accounts, although it may be possible to rename your account to something generic and innocuous when permanently leaving the project (meta:Courtesy vanishing).