how to freeze your puregym membership if you have a fixed-term / annual membership

You can't

"

1. If you have purchased a Fixed Term membership you will be entitled to 50% of the pro rata’d amount as refund for any unused membership.
2. You cannot freeze your membership on fixed term membership.

"

See this link:
https://www.puregym.com/membership-terms-conditions/#:~:text=You%20cannot%20freeze%20your%20membership%20on%20fixed%20term%20membership.