I'm not sure if they offer firmware updates for the P990 but start at www.sonyericsson.com and go to the support pages. You can also do a full reset from your options which will erase eerything on your phone.
First try a reset from the options menu on your phone.
From the website check if they offer the Update Service for your phone then you can do it at home. Otherwise you might try contacting Sony Ericsson directly for help.