Run the application `./electrum-abc`, go to `Tools > Preferences > Fiat` select a fiat currency that was never selected before, check the `Show history rates` checkbox.
Check that a new file was created for this currency in `~/.electrum-abc/cache` and is complete.
Close Electrum ABC, edit the cache file (remove the last few entries) and save it, make sure the cached data file is older than 24 hours (`touch -d "2 days ago" ~/.electrum-abc/cache/CoinGecko_GBP`) so that the API call is triggered on the next restart of the application. Restart Electrum ABC, check that the cache file get updated with the previously removed entries).