I recently moved my website from HTTP to HTTPS. But when I try to get my HTTPS pages indexed, Google search console says "Duplicate, submitted URL not selected as canonical."
Google is preferring the HTTP version and indexing it but not the HTTPS. I tried adding user-declared canonical using the Yoast plugin but still, it is not getting indexed.
Should I redirect my entire website to HTTPS by editing the .htaccess file or is there any easier solution?
You should leave the canonical tags in place with Yoast and hold off on doing redirects. Unfortunately, it will take up to a year for Google to index all your pages on HTTPS and there doesn't appear to be a way to speed that up that doesn't risk ranking drops.
When I migrated my largest site to HTTPS, I used canonical tags to do so. It took a year, but otherwise the process went smoothly. See my answer to HTTP to HTTPS: Wait for new sitemap to be indexed?
Implementing redirects from HTTP to HTTPS seems to cause ranking drops. See Are drops in Google ranking common after switching to https?. I would hold off on the redirects until Google indexes your HTTPS site. Then it would be safe to put in the redirects.