Title says it. How can I upgrade while staying on LTS ?
GNOME (and previously, Unity) versions on Ubuntu LTS (and even non-LTS) typically stick to whatever version they were at release, with only security updates applied. There will also hardly ever be a PPA for a newer GNOME version because of the sheer amount of work involved in building it and maintaining it for each new GNOME version. The only viable option to use GNOME 3.32 (or newer) on Ubuntu is to update to 19.04 or newer as it becomes available.