1) create a copy of your current project
2) Download the requiered version of the Mendix modeller
3) Open the copy project in Mx 7.7.1. (Select open from disk and browse to the correct folder.
4) the project will be updated.
5) check if everything works (custom widgets/java etc).
6) If you are sure everything works do the samesame with your original project and commit to the teamserver.
Be aware that once you have updated, reverting to an older version is no longer possible.