Go to the catalogue, identify yourself with your ULL card number and your password and then click "Mi biblioteca / My library"
You'll see the documents yo've got on loan. Check the document you want to renew and click "renovar / renew".
As a rule, you can't renew a loan more than twice, though some types of documents may have different renewal conditions. If the document you want to renew has been reserved by anothe user, you won't be able to renew your loan.
Remember that your reader number is the 10-digit number shown below the barcode at the back of your ULL ID card, and your password is your birthdate in the form DDMMYYY (e.g.., if you were born on 15 March 1998 yor password is 15031998)