1. Select "Replace String in Comment" from More actions
User must have Edit All Comments or Edit Own Comments project permissions for selected issue's project to access/see "Replace String in Comment" action at More action list.
2. Select new and old text to replace string in comments
If user has Edit All Comments project permissions for selected issue, s/he can select all comments.
If user has only Edit Own Comments project permissions for selected issue, s/he can only select own comments.
3. Confirm to replace string in comments