Thank you for your answer.
“FP3” appears under “this PC” in Explorer.
I have now followed these instructions ✏ FP3/FP3+: Complete guide for installing LineageOS
I made it to the recovery. In Recovery I clicked on Factory Reset and then format data/factory reset. Then Apply update and then apply from adb.
I then entered adb sideload lineage-18.1-20211004-nightly-PF3-signed.zip in the Minimal ADB and Fastboot window/terminal.
Now everything works. The terminal says: adb: failed to stat file lineage-18.1-20211004-nightly-PF3-signed.zip: No such file or directory.
And nothing happens.
The mobile says:
Recovery
Version 18.1 (20211004)
Active slot: b
ADB Sideload
Cancel