To change it system-wide, go to your coresettings.config file and change this line:
<customSetting key="ReportMaxRowCount" type="local" value="50000" />
It can be found in C:\ProgramData\Symantec\SMP\Settings.
If you want to change that limitation for a single report, you will have to right click and choose Export. Then, find the embedded SQL and add this line at the very top:
SET ROWCOUNT 0 --Unlimited
If the report is simply running a stored procedure, then you will have to go into Management Studio and modify the stored proc directly and add that same line above.