Just one thing: are you sure that, in this particular piece, it is a slur and not a phrase marking? If it’s just a phrase marking, it just means that the section is played legato without even a break for breathing.
There are several ways to tell the difference, but the one that seems the most common in my memory is that a phrase marking will often not actually touch the starting and ending notes, and there will likely be a bunch of them throughout the piece, with few or any notes not under them (unless there is a sim. marking.)
If you are certain it’s a slur, then I agree with everyone else: break the air slightly either with a slight tonguing or by restricting the air (idealy with the diaphragm.) You also may want to experiment with decreasing the volume of the end of the first note, and then suddenly increasing to start the second. The point is that it must sound like two distinct notes while still sounding like a slur.
Heck, depending on your piece and proficiency, you may want to experiment with lightly tonguing all notes in a slur. I’ve heard it done before, and I know you have to with some instruments.