"Sion" with an "s" only appears twice in the Old Testament and not in all English translations as I'll show in just a moment. The first place which it appears in the KJV is here:
"From Aroer, which is by the bank of the river Arnon, even unto mount Sion, which is Hermon," (Deuteronomy 4:48)
This "mount Sion" is NOT the Mt. Zion which we read of elsewhere all throughout the Old Testament in that it is actually Mt. Hermon. Also, the underlying Hebrew word "Siy'on"...
Hebrew Lexicon :: H7865 (KJV)
...which is here translated as "Sion" is NOT the same underlying Hebrew word which is elsewhere translated as "Zion" all throughout the Old Testament. In this instance, the Hebrew word "Siy'on" simply means "lofty" so it was used in a descriptive sense in relation to the "lofty" Mt. Hermon.
The only other place where the word "Sion" appears in the Old Testament of the KJV is here:
"Praise waiteth for thee, O God, in Sion: and unto thee shall the vow be performed." (Psalm 65:1)
This "Sion" is definitely the same "Zion" or "Mt. Zion" which we read of all throughout the Old Testament. Here, the underlying Hebrew word is "Tsiyown"...
Hebrew Lexicon :: H6726 (KJV)
...and it appears a total of 154 times in the underlying Hebrew of the Old Testament and in 153 or those 154 times it is translated as "Zion" in the KJV and Psalm 65:1 is the sole exception where it is translated as "Sion". Why the change in spelling by the KJV translators? Honestly, I have no idea whatsoever, but, again, it is spelled with a "Z" in many other English translations in Psalm 65:1 as you can see here (click on the verse below and it will take you to a site with translations of this verse in different Bible versions):
Psalm 65:1 Praise awaits you, our God, in Zion; to you our vows will be fulfilled.
Only because I received an email from a damsel in distress...