Use xtickangle(), eg:
A = 5:5:25;
B = {'A1=5', 'A2=10', 'A3=15', 'A4=20', 'A5=25'};
figure(1)
bar(1:5, A, 'facecolor', 'm');
xticks(1:5)
xticklabels({'A1=5', 'A2=10', 'A3=15', 'A4=20', 'A5=25'})
xtickangle(30)
figure(2)
bar(1:5, A, 'facecolor', 'm');
xticks(1:5)
xticklabels({'A1=5', 'A2=10', 'A3=15', 'A4=20', 'A5=25'})
xtickangle(45)
figure(3)
bar(1:5, A, 'facecolor', 'm');
xticks(1:5)
xticklabels({'A1=5', 'A2=10', 'A3=15', 'A4=20', 'A5=25'})
xtickangle(90)
figure(4)
bar(1:5, A, 'facecolor', 'm');
xticks(1:5)
xticklabels({'A1=5', 'A2=10', 'A3=15', 'A4=20', 'A5=25'})
xtickangle(0)