how do I save or export to an exell file the history trades of back testing?


One way of achieving this is to write some additional software: every time when a trade is performed it writes/appends the relevant information to a csv file.

Starting point for the reference manual is here:

Once your backtest has completed you can go to the correct directory and find the csv file with all trade information there. This file can be read with Excel.

Documentation on MQL5: File Functions
Documentation on MQL5: File Functions
For security reasons, work with files is strictly controlled in the MQL5 language. Files with which file operations are conducted using MQL5 means cannot be outside the file sandbox. the common folder for all the terminals installed on a computer - usually located in the directory C:\Documents and Settings\All Users\Application...