You need to make a NSMutableAttributedString string and then assign it. So for more info see in this example.
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = @"Cell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier forIndexPath:indexPath];
// Configure the cell...
NSMutableAttributedString *str=[[NSMutableAttributedString alloc] initWithString:[infoCells objectAtIndex:indexPath.row]];
[str addAttribute:NSFontAttributeName value:[UIFont fontWithName:@"Helvetica-Bold" size:50] range:NSMakeRange(0, str.length)];
cell.textLabel.attributedText = str;
//cell.textLabel.font = [UIFont fontWithName:@"Adobe Arabic" size:20];
return cell;
}