If you wish to sign in as a different person, go to Help->Sync and choose the logout button. When asked choose logout completely. You can then click the button to sign in as a different person.

Note that when you sign in, all the data in OfficeTime is uploaded and synced. If you don’t wish that to happen, then start with a blank file. Choose File->New File, give it any name. Then choose Help-Sync and sign in.

You cannot change who is signed in on the iPhone/iPad version.