Add autonumber to a large table?

[Accessnovice]

Hi,

I am new to Access 2010 and would like to add autonumber to a large existing table (>2million records and 25 fields).

I have tried in design view with adding row then selecting autonumber as datatype, however, an error message appears:

"the query cannot be completed. either the size of the query result is larger than the maximum size of a database (2 GB), or there is not enough temporary storage space on the disk to store the query result."

I can't understand this as the database is only ~300kb and I have 8 GB ram?

Any ideas?

Many thanks!