• 改札かいさつ — ticket inspection gate
• 改革かいかく — reform
• 改善かいぜん — improvement
• 改良かいりょう — refinement
• 改正かいせい — revision
• 改あらためる — to change, to revise
• 改あらたまる — to be renewed, to be corrected

The character 改 combines 己 (self) with 攵 (action), suggesting the act of reforming oneself or inspecting actions. It’s commonly used in government, transportation, and personal improvement contexts.

考かんがえを 改あらためた。
I changed my opinion.

電車でんしゃに 乗のる前まえに 改札かいさつを 通とおってください。
Please go through the ticket gate before boarding the train.

カイ is the on-yomi, often used in compound words. The kun-yomi forms あらためる (to revise/change) and あらたまる (to be changed) are used in more direct, active expressions.