How to upgrade operators' skills?

How can I upgrade my Operator's skills in Arknights? Skills can be upgraded as long as you have completed the item requirements and currency that's needed. To check the current level of your Operator's skills, you can check their information through Operator Management.

You can upgrade the skills by pressing the skill level on the left side. As long as you have a "Skill Summary" item, you can level up your skills. In order to obtain new skills on the other hand requires you to promote your character, so check the promotion section in order to see what skills you can obtain.