Removing a particular integration (like Strava) will remove the data from that source, in the case of something going wrong with importing and if you need a fresh start.
The app is optimized for retaining data and building an accurate history, so removing old data for particular time periods is not currently possible. The reports and health score constantly recalculate, so we suggest not worrying about deleting an older section of time that was "not ideal" etc. and just continuing from today onwards. Even if there was a point in time when you had less activity or were less healthy, that is fine to keep in the account and shouldn't be deleted.
The reports tab will show the last 10 days of history, so anything previous from that has little impact on the app experience. The Health Score takes into account the recent few months of data when available, so that should also be constantly refreshing on a rolling basis.
Individual data points for certain types (like weight, workout, meditation, etc.) where a wrong item may have been uploaded can also be removed in the app, from the Trends page > data tab.
You can always delete your account if you no longer wish the data to be stored and do not plan to come back. It can be done at https://gyrosco.pe/delete/ — however that permanently removes the account and associated integrations, so reimporting the same data later may not work.