How to accomplish this using ONLY one update sentence?

There is a table named test1.
it have 2 number fields: field1 field2
the initial value is:
field1 field2

1      null
1      null
2      null
3      null
3      null
3      null

how to accomplish this using ONLY one update sentence? Group by field1, that is field1 =1 is one group, field2 =2 is another group use field2 to represent the sequence in the group

the result is :

field1 field2

1      1
1      2
2      1
3      1
3      2
3      3