For future samples, you can easily discard some measurements already in the collector by adjusting the
measurementUpdateLimit configuration property in ruuvi-collector.properties.
Custom retention policies and continuous queries for downsampling the data is another more advanced approach which lets you “gradually” downsample data over time as it gets older.
For downsampling the existing data, you need to run an “INTO query” as mentioned in this post (obviously do this before setting the retention policies to drop the oldest “full accuracy” data if you don’t want to lose historic data. Though as always, remember to take backups before experimenting)