it works. my engl. sucks but I try my best to explain.
go to your 'homepage', then 'I already read', on the right site you can see all your taggs. or just click on your manga wich you just tagged and click on the tag you want to delete. then you see all mangas with this unwanted tag. now its bothersome... you have to click on every manga and edit, erase the tag (on every manga) and dont forget to 'refresh' mangago. you can see everytime you delete/erase the tag the number is shrinking. sometimes it happends that you see a '0' on your unwanted tag. then just click on it. you will see there will be no manga- then refresh and its gone.
na not delete the manga. only the wrong tag. I just made a screenshot album. may it helps a bit better. http://www.mangago.me/home/album/58481/
but I have to be honest first I also thought it doesnt work anymore. but it was bec. I was too impatient. even if you delete and refresh- it will still be there but after few minutes... suddenly its gone... and thanks! I am happy that you could understand me :)
I just tried again. for me it works. I will explain what I do: go to your homepage, then 'already read' - now you should see all your tags on the right site. click on the tag you want to delete. then you can see all mangas you tagged with the tag you want to delete. (now its a bit bothersome) below every manga you have your tags, also your summary and "edit" + "delete". click on edit and just erase your unwanted tag (click on it and it will disapear) then click "submit". then the former page appears again (remember all mangas with your unwanted tags) next manga and next. sometimes it needs a bit time for the last one. like you already erased the tag on the last one and on your list you can still see it. just give it some time (guess the programm just need some time) it works! it cant be that it only works for me and nobody else ╮( ̄▽ ̄)╭
for me it works like I described it back than. on your homepage, when you click on mangas, you can see all your tags on the right site. then click on the tag you want to remove. you will see all the mangas with your tag for example 'red hair'. the heading is 'red hair' with the number how many mangas you have with this tag. you have to go on every single manga (edit) and remove the tag and safe. you will see that the number is shrinking. then there will be the last one and you will think it doesn work bec. 1 is still there... back to your homepage, tag list- now by 'red hair' is 1 but when you click on it there is 0 and then its gone- at least, that is exactly what I was doing a minute ago.
I can confirm sometimes tags will be stubborn and can't be deleted. I have a tag that I no longer need which has 29 mangas in it. And when I try removing that specific tag from the 29 mangas, only 23 were succesfully. The other 6 is stuck with that tag after the tag removal. I've tried deleting it from my account altogether. I've tried waiting for days (in case it needs time to renew information). It just... won't get removed. Now IDK what to do to delete that tag off my account.

I mean completely deleting it. I'm trying to organize my manga list, so I've tried taking it out of each manga and also deleting the manga itself but it is still somehow there. Is it a glitch, or it's just hopeless?