On the Card in use the three dots ... to bring up the Action Menu.
If the the issue is awaiting a JIRA service desk approval, then the option to Approve / Decline will appear.
- Approve - will perform the approve action as specified in the workflow.
- Decline - will perform the decline action as specified in the workflow.
To configure approvals follow the guide Atlassian is providing: https://confluence.atlassian.com/adminjiraserver/configuring-jira-service-desk-approvals-938847527.html