On the source issue click on the Actions (three dots) dropdown. Select Merge to.
2. Select target issues and choose what fields and custom fields will be merged. You can choose as many as you want. You can also adjust the merge action by settings other options.
Comments from source issue will be copied to target issues
All attachments from source issue will be copied to target issues
All links from source issue will be copied to target issues
Edit Issue trigger will be dispatched
Sync next comments
Next comments from source issue will be added to target issue automatically. More information is here
Link source and target issue with selected issue link type
3. Click on the Merge link
4. On the last window you can follow the status of the merging issues. You might see warnings and errors with additional explanations. You can close this window any time with no impact on the merge process.