From the tags index find the first tag you wish to merge, click on the dropdown menu button beside it and select the Mark for merging option. Next find the second tag you wish to merge, click on the dropdown menu button beside it and select the ‘Merge tags’ option.
A menu will now appear where you can choose which of the tags you wish to keep. Once you have chosen, the two tags will be merged and you can then edit the combined tag further if required.