(2-part post):
First point::
You've written that you 'have properties together'. There are two ways of jointly owning a property. If you're 'tenants in common', you each own a specific share of the property. (The distribution would be 50:50 unless otherwise specified in the deeds). However, if you're 'joint tenants' then neither of you owns a specific portion of the property; it's your 'legal partnership' which owns the entire property.
When one of you dies, it makes a big difference as to which type of ownership you have. If you're tenants in common, the deceased person's share of the property will go to whoever they've left it to in their will. (If they've not left a valid will, the intestacy rules will apply).
However,
if you're joint tenants, the surviving partner automatically acquires ownership of the entire property. That rule takes precedence over the provisions of any will. (If you're a joint tenant you can't leave your 'share' of a property to anyone, since you don't actually own a specific share).
Second point:
A will normally remains valid until it is revoked by the testator. (See below for very important exceptions). It can, of course, be modified by adding a codicil (although it's usually just as easy, and often clearer, to draft a fresh will). One way to revoke a will is through deliberate destruction:
http://www.hmrc.gov.uk/manuals/ihtmanual/IHTM1 2073.htm
Alternatively, it can be revoked by writing a new will (which would normally commence with a clause revoking all earlier wills).