Can I reset cursor's position to the beginning?

you need to declare your cursor as scroll, like this

declare c scroll cursor for (select statement); 

then at any time for locating to the first just use the following

fetch first from c;

Another option that can be used that doesn't force you to change the type of cursor is simply to close the cursor and re-open it:

CLOSE user_cursor
OPEN user_cursor

But the scroll option will be cheaper in terms of resource usage, if that's a change you can accept.