Consider what thou owest to his immutability. Though thou hast changed a thousand times, he has not changed once; though thou hast shifted thy intentions, and thy will, yet he has not once swerved from his eternal purpose, but still has held thee fast. - Charles Spurgeon  The immutability of God means that God is unchanging. More specifically, "God is unchanging in his character, will, and covenant promises.
systematic theology text (a Reformed classic) defines God's immutability as 'that perfection of God by which He does not change in His being, perfections, purposes, or promises.'
Westminster Shorter Catechism says, 'God is a spirit, whose being, wisdom power, holiness, justice, goodness, and truth are infinite, eternal, and unchangeable." Those things do not change. A number
of Scriptures attest to this idea (e.g. Num. 23:19; 1 Sam. 15:29; Ps. 102:26; Mal. 3:6; 2 Tim. 2:13; Heb. 6:17-18; Jam. 1:17)."
Does God change his mind?
"On several occasions, the Scriptures speak of God 'repenting' or 'changing His mind' (see Genesis 6:5-6; Exodus 32:14; Jonah 3:10; 2 Samuel 24:16). Do such texts undermine our confidence in the immutability of God? Most certainly
not! First, we must clarify what immutability means. Immutability applies to the nature of God. He is always God, and He is always infinitely powerful. Never will God fail to accomplish His will due to a change in His power to accomplish His purposes.
Second, God is immutable with regard to His character or attributes:
'... God is immutable in His attributes. Whatever the attributes of God were before the universe was called into existence, they are precisely the same now, and will remain so forever. Necessarily so; for they are the very perfections, the essential
qualities of His being. Semper idem (always the same) is written across every one of them. His power is unabated, His wisdom undiminished, His holiness unsullied. The attributes of God can no more change than deity can cease to be. His veracity is immutable,
for His Word is ‘forever settled in heaven’ (Ps 119:89). His love is eternal: ‘I have loved thee with an everlasting love’ (Jer 31:3) and, ‘Having loved his own which were in the world, he loved them unto the end’
(Jn 13:1). His mercy ceases not, for it is ‘everlasting’ (Ps l00:5).' (Arthur W. Pink, Gleanings in the Godhead (Chicago: Moody Press, 1975), pp. 35-36)