On that motherboard the best you can do is an i7 7700K, but it won't be overclockable, so might as well go with 7700 unless you want to upgrade the motherboard as well, in which case it would be better to either go for 10th gen Intel or AMD Ryzen. This is, of course, assuming that it is a standard size motherboard, not sized specially for the AIO system. Use CPU-Z to determine the exact model number of the motherboard, and see if it is a standard size board or not. This is the major part that will hold you back on a full system upgrade, if the motherboard is non-standard you cannot upgrade it since the newer motherboard might not fit your system.
Assuming a non-standard size motherboard, best upgrade you can go for is an i7 7700, add another 8 GB of RAM for 16 GB, and get a mid-range GPU like a GTX 1660 or 1660 Super(assuming the PSU can handle it).