The weighted data for each survey is supposed to yield a total population estimate. When you combine x number of surveys you will get roughly x*the population total.
You have to figure out the contribution of each cycle (i.e. total weight) and adjust as a proportion of the desired population year.
Likely by one or more population characteristics such as age, sex and geography. As a minimum they should be the Strata and Cluster variables. And hope the cluster definitions are the same or you have to standardize your data to one of the clusters so the data can be combined correctly.
You should pick one of the years as the standard to adjust to. There's a number of arguments about whether to use the middle value, if available vs the end points.
There are some details specific to NHANES at https://wwwn.cdc.gov/nchs/nhanes/tutorials/Weighting.aspx that specifically mentions multiple cycles. Follow the links.
Google may be your friend sometimes.
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