How do you open a single lever tap??

Question

I've taken the plastic cover off the front to find a hole but can't open it using an allen key

Answer 1

how many keys in your set of allen keys?  Allen keys come in all sizes a good sized mechanics set may have 16 to 32 different sizes.

Many sets on a key ring have 6 to 8 sizes.  Why do you need to open the lever?  If it is not working you may need to replace it.  If it just seems clogged try cleaning the screen part of the "bubbler" (aereator)

and brush it out with a tooth brush.  remove carefully and reassemble it the same way... or purchase a new one.  they are inexpensive  take the old one to be sure you get a perfect fit. good luck   a homeowner will usually need to become something of a plumber ask at home depot, lowes, or other store they will usually be able to help you.