Domino 4.4 and above
Once a Domino git based project is created, the Git code repository it is based on cannot be changed from the Domino UI.
Git based projects also cannot be converted to a regular Domino File System projects.
If you made an error creating your project, simply archive the current project from the project settings and create a new project with the correct Git Repo information. If you have a use case that makes a new project particularly difficult, as in impact to difficult to change upstream workflows, the repo can be updated via the mongo database. Note that edits to the mongodb are inherently dangerous and this is not recommended unless a new project is really problematic. Reach out to Domino Support if you have questions.
There is a feature request to allow the repo backing the git repository to be changed with engineering.