Omnummeren means to renumber.
In the past, addresses often changed from one census/population register to the next. The houses would be omgenummerd; renumbered. This can make it difficult to find the modern locations of historical addresses. See tips for finding the address where your ancestors lived for some ideas.
Some archives have omnummerlijsten (renumber lists) or omnummerregisters (renumber registers), either created in the past and part of their records, or created in the modern period as an aid to researchers. For example, the Amsterdam City Archives publishes omnummerregisters for Amsterdam addresses.