how to expand space for more softwares

[Tapas Mishra]

I keep installing various softwares on my Ubuntu desktop.Right now I
am stuck up with space problem.
I have some partitions which are being used by other OS but they have
plenty of space free.
It is mountable/read/write happens.
I am not clear if I have to expand space for more installations then
/opt
/usr or /lib or /etc which directory and how should I go ahead to give
them more space.

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