Help how explain . / source work

[Budi]

Help to have . / source work to perform total replacement of current
loaded/run sourced codes script as e.g.

$ , bashscript

just add commands/codes script to the  current loaded/run sourced one,
while may replace if only it is coincidentally the same identifier
name

But so explain how to have total replacing it, i.e. clean up the
current loaded/run source before sourcing the new