Sorry, I will not do your homework and please don't blame me for your tight schedule.
I will show you how to do it yourself.
In your schematic, U8B and U23B are set to binary 11 = decimal 3 upon reset (at least by the simulator - a real circuit will behave differently, as I explained above).
- Remove the fixed connection from all the CLR and SET inputs of the counter flipflops to VCC (top left).
- find the required states for all flipflops after reset to display 15.
- create a reset line, low active. This line should be High if inactive. Use e.g. a pull-up resistor to VCC.
- connect the SET and CLR lines off all flipflops such that
- if a flipflop needs to be set, connect CLR to VCC using a pull-up resistor, connect SET to the reset line
- if a flipflop needs to be cleared, connect CLR to the reset line, connect SET to VCC using a pull-up resistor.
- Connect the reset line to a low-active signal, e.g. from a switch.