Clarify updating existing data

docs/backtesting.md

@@ -7,7 +7,7 @@ Backtesting.
 
 To download data (candles / OHLCV) needed for backtesting and hyperoptimization use the `freqtrade download-data` command.
 
-If no additional parameter is specified, freqtrade will download data for `"1m"` and `"5m"` timeframes.
+If no additional parameter is specified, freqtrade will download data for `"1m"` and `"5m"` timeframes for 30 days.
 Exchange and pairs will come from `config.json` (if specified using `-c/--config`). Otherwise `--exchange` becomes mandatory.
 
 Alternatively, a `pairs.json` file can be used.
@@ -37,6 +37,10 @@ This will download ticker data for all the currency pairs you defined in `pairs.
 - Use `--timeframes` to specify which tickers to download. Default is `--timeframes 1m 5m` which will download 1-minute and 5-minute tickers.
 - To use exchange, timeframe and list of pairs as defined in your configuration file, use the `-c/--config` option. With this, the script uses the whitelist defined in the config as the list of currency pairs to download data for and does not require the pairs.json file. You can combine `-c/--config` with most other options.
 
+!!! Tip Updating existing data
+    If you already have backtesting data available in your data-directory and would like to refresh this data up to today, use `--days xx` with a number slightly higher than the missing number of days. Freqtrade will load the available data and only download the missing data.
+    Be carefull though: If the number is too small (which would result in a few missing days), the whole dataset will be removed and only xx days will be downloaded.
+
 ## Test your strategy with Backtesting
 
 Now you have good Buy and Sell strategies and some historic data, you want to test it against