Hi; The simplest and easiest way , is to go under "Developer mode", and then open the form (you want), open the drop-down menu "Debug view" and choose "Set Defaults", modify the default values as you need.

steps to go under "Developer mode" and change default values (example of "Emplyee form"):

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The more complexe way is to find the file where the field you want to change the default value is defined (created) [Probably in a class a_class definition after _columns = {.......} creation in _defaults = {..somewhere...here ..the default value....} statement, as in the example below ( extracted from the code mrp.py file of the mrp module)]:

Hope this can help, and best regards.