I'm thinking you don't have enough room to put your foot on the tread (front to back). To make that wider or bigger you will need to make a new staricase altogether.
Stairs have a riser and a tread, refered to as rise and run. To make the tread larger the stairs have to stick out in the room further. For instance, if you want a 1' wide tread and there are 12 steps, the stairs would have to have 12' of floor space from a plumb line from the terminus at the top to the bottom tread.
It is a major rework and seldom worth the trouble, expense and sometimes there is just not enough space left for it.