Manual merge

Steps

On the source issue click on the More dropdown. Then select Merge.

When the Merge Dialog opens, 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.

Info

If you want to allow to select only one target issue, uncheck the Allows merge to many issues option in the project settings.

Info

This step is enabled by default, however field selection is skipped when Allows to select fields by user is disabled in the project settings. Fields selected in Default fields to merge option will be used or when empty then fields from global settings Fields from Merge scope.

You can enable Sync next comment. Read more about this feature here.
To finish the merge, click on the Merge button.