If you are on .NET Framework 4.5.2, 4.6 or 4.6.1, you are in the final year to upgrade. Updates cease April 2022; this includes technical support and security fixes.
Minimally you need to upgrade to 4.6.2, but with .NET Framework 4.8 out, why not take the time to jump to the most modern form available?
This Microsoft blog provides insight into the reason behind the end of support and why it is beneficial, particularly for security, to upgrade now.
Your applications running on these versions will continue to work, but to upgrade, simply update the runtime to minimally 4.6.2 to ensure security and support. In most cases, you won’t need to retarget or recompile, though it is always recommended you validate all the functionality before deploying any updates.