Hi @Talingar,
Welcome to the :qiime2: forum.
You may want to look at why your reference library didn't work so well, or try a different techique for assignment. (Others are probably better equiped to help with that, Ive not done COI). However, you could export your taxonomy.qza artifact to a text file (see the exporting data page), use qiime2R to import it into R or use the python API to pull it into python and make your edits there. (My personal recommendation is python or R because then you have a better record of what got changed where and why and how.)
If you go with the R or export solution, you then need to re-import the updated taxonomy as an artifact.
Best,
Justine