It seems trite to say “pass it to someone else”, but that’s most likely what you need to do. If there’s someone who you know can do it, then see if you can pass it to them. On the other hand, if it’s important, and it’s a core part of your work, this might not be possible. So, make use of your network and ask for advice. If there’s no-one who can advise you, then it may be worth asking your manager for their advice. Mind you, they might not know – maybe that’s why they passed the task to you. But asking is a good idea; even if nothing else, it lets them know that you’re on the case, even if you’re struggling.
Remember that this issue in reverse can affect people that you pass work to. It’s human nature for people to do first the things that they enjoy doing, and so unfamiliar or disliked tasks may sit in someone’s ‘In’ tray. The person may well revisit the task from time to time, but they may find other, more interesting, work to do. So, monitor the situation, and talk. Asking in person is always better than writing – after all, if you ask in writing, your question may be pushed down the priority list as well. It just adds to the workload. Also, if you ask in person, then you get to see how comfortable the person is from their answer.