There is no "downgrade" option available, the only solution is to uninstall/reinstall. Note that any work saved in 7.5 can most likely not be opened in an earlier version.
Before uninstalling you could try and contact
Reallusion Support and provide then with full details of the issues that you are having.
Gerry
System: Win 10 Pro (21H2), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver Studio-536.99), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.