It may be possible to resit an examination, or re-enter another piece of assessment (e.g. essays, reports, dissertations), if you do not complete it satisfactorily.
Your handbook and programme specifications for the regulations applying to your programme of study should tell you whether it is possible.
The My Birkbeck fees section displays a table of fees that may be applicable. The fees section can be found here.
For module Programmes, the My Birkbeck website contains useful guidance on reassessment which can be found here.