There are several errors that you're making.
For a start, () indexing on tables return tables. You would have to use {} to actually get to the content of the column. So:
data{ii, 1}
Secondly, you cannot use == to compare strings. You have to use strcmp. So:
if strcmp(data{ii, 1}, 'Arsenal')
But possibly, the biggest error, is to use a loop and lots of if to do something that could be done in only two lines:
[~, ~, uids] = unique(data{:, 1}); %change the strings into unique ids alphabetically
data.(data.Properties.VariableNames{1}) = uids;
Note that if you use column names it's even simpler:
[~, ~, uids] = unique(data.NameOfFirstColumn);
data.NameOfFirstColumn = uids;
