To delete a skill :
Manage>Skills
On the Skills tab- click on the 3 dots next to the skill you wish to delete

3. Click on Delete.
Please note that if you delete a skill it will be removed from ALL resources. Consider using the remove option as well- instructions are in the following article:
https://help.runn.io/en/articles/6378464-how-do-i-remove-a-skill-from-a-person