This can be changed in Page Properties https://docs.mendix.com/refguide/page-properties/#parameters
Right click somewhere in your Page Editor, outside of the Page itself (so on a greyed out Layout piece for example), go to Properties and change the Page title.
Go to page properties and rename title field.
like below image
Do you have a multilingual app? If yes, it could be the issue that you still need to change the title for the other languages your app supports.