DIFFERENCE BETWEEN SUBSCRIPTION AND UPDATE ON AN ANTIVIRUS SOFTWARE

A subscription according to Oxford Advanced Learner's Dictionary is an amount of money you pay usually once a year to renew a copy or copies of something. In this case it can be said to be the money paid to use the antivirus for a particular period of time. This can be for a year, two or more. The number of years for which the user subscribed to the antivirus is usually shown on the GUI( Graphical User Interface)of the antivirus counting down to zero. When you subscription expires, the whole antivirus function shut down.
An update according to Oxford Advanced Learners Dictionary means to make something more modern by adding new parts. In an antivirus, updating it downloads and incorporates latest antivirus definitions ( Defence Mechanism) into the antivirus in question. This will help protect you against newly discovered viruses.
Here is an analogy to drive my point home. What we described above is like paying for a house rent and renovating the house after sometime. You pay a house rent to stay in the house for 2 years( You just subscribed to use the house for 2 years). When you renovate the house after 1 year of your stay there to make it look like the other houses(You just updated the house)
Note: Anti-viruses are meant to be updated daily. This is because newer viruses are programmed on daily basis and antivirus companies discover them and bring up solutions which you can only get by updating your antivirus.