Understanding uncertainty of thoughts and the mind It's quite easy to see the changing behaviour of the things we see, hear and feel.

Appearence between a child and an old man.
Good smell and bad smell
Having a wound and not

Those things are quite easy to see. But it is hard to see the changing behaviour of mind and thoughts.
I'm aware of

I may angry now. But I will be grateful in next five minutes. (Changing thoughts). (But I'm also getting repitative thoughts)
I have different prespective than in my childhood.
Knowledge that I have is changing.
If cut down the body into pieces, we can not find any &quot;I&quot;
I and I in the previous life shoud be totaly different (Can see this clearly if I was an animal.)

Even thoughts are changing, It feels like source of it remains unchanged. How to understand this clearly? What are the practises that I should do? What meditation practises help to understand this?
What you describe is the phenomena of the monkey-like mind, which changes more quickly than the body.
From SN 12.61:

“It would be better, bhikkhus, for the uninstructed worldling to take
as self this body composed of the four great elements rather than the
mind. For what reason? Because this body composed of the four great
elements is seen standing for one year, for two years, for three,
four, five, or ten years, for twenty, thirty, forty, or fifty years,
for a hundred years, or even longer. But that which is called ‘mind’
and ‘mentality’ and ‘consciousness’ arises as one thing and ceases as
another by day and by night. Just as a monkey roaming through a forest
grabs hold of one branch, lets that go and grabs another, then lets
that go and grabs still another, so too that which is called ‘mind’
and ‘mentality’ and ‘consciousness’ arises as one thing and ceases as
another by day and by night.
“Therein, bhikkhus, the instructed noble disciple attends closely and
carefully to dependent origination itself thus: ‘When this exists,
that comes to be; with the arising of this, that arises. When this
does not exist, that does not come to be; with the cessation of this,
that ceases. That is, with ignorance as condition, volitional
formations come to be; with volitional formations as condition,
consciousness…. Such is the origin of this whole mass of suffering.
But with the remainderless fading away and cessation of ignorance
comes cessation of volitional formations; with the cessation of
volitional formations, cessation of consciousness…. Such is the
cessation of this whole mass of suffering.

OP wrote:

Even thoughts are changing, It feels like source of it remains
unchanged. How to understand this clearly? What are the practises that
I should do? What meditation practises help to understand this?

Please try the meditation practice taught by Ven. Yuttadhammo in his booklet &quot;How To Meditate&quot; and also taught in this video playlist.
I quote from Chapter One of &quot;How To Meditate&quot;:

The basic technique of meditation that we use to facilitate this
change is the creation of clear awareness. In meditation, we try to
create a clear awareness of every experience as it occurs. Without
meditating, we tend to immediately judge and react to our experiences
as “good”, “bad”, “me”, “mine”, etc., which in turn gives rise to
stress, suffering, and mental sickness. By creating a clear thought
about the object, we replace these sort of judgements with a simple
recognition of the object as it is.