Hi there,
“How do I change the font type in the menu and in the current text, its size without altering the aspect ratio for PC or smartphone?”
You can change the font via template options -> typography
“How I change the words in “Let’s Talk To Us” as Your name, your email, Email subject?”
Just go to Quix -> Pages -> Contact Us, take a look the second column (first section), here you can change the Text “Let’s Talk To Us”
If you want to translate the text placeholder for form fields, you can use Joomla language override with the with Language Constant:
QX_CF_YOUR_NAME
QX_CF_YOUR_EMAIL
QX_CF_YOUR_SUBJECT
QX_CF_YOUR_MESSAGE
QX_CF_SEND