Some assessments are repeatable, others are not. You can check this in the overview of the completed assessments. There is a column titled "repeatable at", which either says "not repeatable" or shows a specific date. If there is a date, you can repeat the assessment once this date has been reached.
Since not all assessments are repeatable and even the repeatable ones have a repeat delay of at least 60 days, you should always try to give your best when you take them. Do not take them when you are in a hurry, unconcentrated, or easily distracted. It is much better to wait for a time you can really concentrate on it and can get the best result possible.