If there is a break clause in your tenancy agreement it is possible to end your tenancy earlier. But break clauses are not a legal requirement so your tenancy agreement may not have one. If this is the case you won't be able to move out before the end of your fixed term unless your landlord agrees to it.
You shouldn't just move out regardless as you'll still be liable to pay rent even after you've left. But fret not, there is another way. You can still move out early if your landlord agrees to this, and after all they are only human. So if you need to move due to sudden change of circumstance they may be willing to let you move out early provided you are up to date with your rent.