It should be fairly simple to remove Mintnav from your Xiaomi device. Open your Google Chrome browser, click on the three dots to open the menu, click Settings, and scroll down to Homepage and click on it. You will see Mintnav set as the site to open when you launch your Chrome browser. To change this, either disable the Homepage altogether or click on Chrome’s homepage. It’s unlikely that an actual infection is on your device, though it’s unclear why your default homepage was changed in the first place. It’s likely that the site does belong to Xiaomi, in which case it should not be dangerous, just very intrusive and annoying to deal with.