I want to set up a VPN client on my LHGG LTE6 kit, which was manufactured in 2021, So I need using RouterOS version 7 or higher. Is it possible to upgrade my RouterOS?
Normally should not be a problem.
True. Assuming it has V6, there are few intermediate to get to "stable". So you'll have to do the /system/package update several times.
Also make sure you update the /system/routerboard firmware too and LTE modem firmware too.
I advise against it, especially the latest version.
And what is this VPN that RouterOS v6 can't connect to?
You didn't mention it; maybe v7 can't do it either.
Upgrading from 6.x to 7.xx (stable) has an intermediate step, you first will upgrade from v 6.x to 7.12.1 (if I recall correctly) and from that you can go upwards.
DO NOT[1] update to the release you will see as latest stable (7.20.4).
Once you get to 7.12.1, decide if it is enough to get your VPN working or if you want a more recent version manually upgrade to 7.19.6.
[1] my personal advice, as 7.20.4 is being reported as having multiple issues.
That is a fair point: worth testing at each stop...