Center tags are deprecated. It would be better to use inline CSS instead however you need to use source mode in order to do it. If you open the page in source mode, the start of the table should look something like this:
The method that ChededFriedShrip suggested is this
The <center> is called a center tag and it is a retired feature. That means that web browsers might (although they haven't yet) decide to stop recognizing it. If and when that happens, that method for centering text won't work anymore. The suggested alternative to using center tags is using CSS to specify that the text should be centered. You do that by adding the style attribute to the table itself as I tried to show in my previous reply. One large benefit of using this for a table is that you only need to use the CSS once on the entire table. In comparison, if you use the tags, you would need to place them around the content of each cell individually. So if you have 9 cells, you would need 9 pairs of center tags as opposed to applying CSS to the table as a whole only once.