Put this at the end of any Templater template:
<%*
setTimeout(() => {
app.fileManager.processFrontMatter(tp.config.target_file, frontmatter => {
// Update or add as many fields as you want
frontmatter['Some property'] = 'Some value'
frontmatter['Another property'] = 'Some other value'
// You can even remove properties if you want
delete frontmatter['Unwanted property']
})
}, 200) // the reason for the timeout is to let the template complete first
-%>