This page explains how to merge issues manually. This is a basic feature of Issue Merger app.
How to merge issues
On the Source issue click on Actions menu (three dots). Select Merge action
2. Select Target issue(s) and choose what fields and custom fields will be merged (you might also click Use preset to simplify your actions). You can also choose option from inlines Merge also and/or Actions after merge
3. Click on Merge button
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