if we had to disassemble the piano for every broken string..... heaven forbid! Just remove the action frame. As you will see, you can easily reach, passing behind the keyboard, the lower part where you see the bridge and, beyond that, where you see the pin around which the string passes or attaches (if the strings have single eyelets). I don't know if your piano has single-eyelet strings or not. You should be careful to place a pair of tweezers where the string turns so that you can work comfortably at the top. Pry the broken part on the affected tuning pins and unscrew it about three turns. Wrap the left one first and then the right one as well. Cut the string about three fingers above the pin. Naturally, you must first measure the broken string with a caliper to choose the suitable diameter.
If you find yourself in difficulty, call a technician. Afterwards, you can do the string seating and subsequent tuning yourself.
P.S. If it is a copper-wound string, you must have it remade in a laboratory according to the exact measurements (total diameter, steel diameter, length of the copper winding, distance between the eyelet and the tip of the bridge).
