QFont font = ui->spinBox->font();
font.setCapitalization(QFont::AllUppercase);
ui->spinBox->setFont(font);

eg:
#include
#include
#include
#include

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QWidget w = new QWidget();
QLayout layout = new QHBoxLayout(w);

QSpinBox* spinBox = new QSpinBox(w);
spinBox->setRange(0, 0xFF);
spinBox->setDisplayIntegerBase(16);
QFont font = spinBox->font();
font.setCapitalization(QFont::AllUppercase);
spinBox->setFont(font);

QSpinBox* spinBox2 = new QSpinBox(w);
spinBox2->setRange(0, 0xFF);
spinBox2->setDisplayIntegerBase(16);

spinBox->setValue(0x1a);
spinBox2->setValue(0x1a);

layout->addWidget(spinBox);
layout->addWidget(spinBox2);

w->show();

return a.exec();

}

效果如图所示
可以设置最大范围

如图所示

知识来源

https://mlog.club/article/4802386