The titular phrase should not be replaced by the often nonsensical colloquialism try and when the intention is to demonstrate an attempt made. Prefer We must try to relax to We must try and relax. While the former shows an endeavor to relax, the latter forms a miscue that implies both an attempt to be made (at what is left unsaid) and a call to relax.
Although Bryan A. Garner suggests that try and is a standard idiom in British English, Google’s Ngram Viewer begs to differ—the correct phrase appears six to seven times more frequently in both British and American English.