Hi all.....Im ok with sql but by no means a guru

i have like 100,000 rows of lineitems
example:
invoiceid, lineitemId, squencenum
9999999 11111111 1
9999999 11111181 2
9999999 11111182 3

the Data consistes of Many Invoices and the Lineitem id
is a Unique Id (Most are consequtive but not a Guarantee) and SequenceNum
is 1,2,3, etc of the line items on the invoice

In my Case i have say 100,000 rows , different Invoice numbers with
different number of lineitems
All the LineItems are out of Sync.....

So I need to Update them in the order 1,2,3 etc for there prospective
Invoice number

Help with a Update Scripts would be helpful
Thanks