The screen of my fenix 5S watch flashes slightly. I reset it and always the same problem. Since this problem, the autonomy of my watch is only one day. Do you know how to remedy this problem?

You can try a total reset back to factory defaults plus wiping all of the data: https://support.garmin.com/en-US/?faq=wxPmdnWsev1xGRUf58oDC8&productID=552982&tab=topics

If thaf does notbhelp, and the watch has been in use for a good number of years, the battery will have deteriotated, and replacing becomes necessary. It is do-able on a regular 5 but may not be possible on a 5S. And it can be another problem like a loose connector, failed component or moisture ingress.

Having a repair done for you might be an option, it can get expensive though. Best to phone Garmin support and discuss with them what the out-of-warranty replacment options are or you. But first give the thorough reset a try.