Is there a way to change default font for your application

The answer is no, you cant change apple's systemFont, you need to set the font on your control yourself.

For best way to set a default font for whole iOS app, please check the below SO questions:

Set a default font for whole iOS app?

How to set a custom font for entire iOS app without specifying size

How do I set a custom font for the whole application?

Is there a simple way to set a default font for the whole app?

Define and export (by including a header in files or in precompiled header) a category of UIFont as following:

@implementation UIFont (Utils)

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-protocol-method-implementation"

+ (UIFont *)systemFontOfSize:(CGFloat)size
{
    return [UIFont fontWithName:@"YOUR_TRUETYPE_FONT_NAME_HERE" size:size];
}

+ (UIFont *)lightSystemFontOfSize:(CGFloat)size
{
    return [UIFont fontWithName:@"YOUR_TRUETYPE_FONT_NAME_HERE" size:size];
}

+ (UIFont *)boldSystemFontOfSize:(CGFloat)size
{
    return [UIFont fontWithName:@"YOUR_TRUETYPE_FONT_NAME_HERE" size:size];
}

+ (UIFont *)preferredFontForTextStyle:(NSString *)style
{
    if ([style isEqualToString:UIFontTextStyleBody])
        return [UIFont systemFontOfSize:17];
    if ([style isEqualToString:UIFontTextStyleHeadline])
        return [UIFont boldSystemFontOfSize:17];
    if ([style isEqualToString:UIFontTextStyleSubheadline])
        return [UIFont systemFontOfSize:15];
    if ([style isEqualToString:UIFontTextStyleFootnote])
        return [UIFont systemFontOfSize:13];
    if ([style isEqualToString:UIFontTextStyleCaption1])
        return [UIFont systemFontOfSize:12];
    if ([style isEqualToString:UIFontTextStyleCaption2])
        return [UIFont systemFontOfSize:11];
    return [UIFont systemFontOfSize:17];
}

#pragma clang diagnostic pop

@end

Light variant comes as a useful extra starting with iOS 7. Enjoy! ;)