Don’t spell out: The UN are monitoring the situation, a UN convoy.
For UN sub-agencies, write out the first time, and thereafter use initials. See initialisms. Note that the UNHCR is referred to as the UN Refugee Agency (as The United Nations High Commissioner for Refugees is wordy).
Exception: Unicef and WHO are commonly known agencies, and don't need spelling out.
until, till, ’til
Use until. Use till only for soil or a shop till. Do not use ‘til.
up-to-date/up to date
Use up-to-date when it precedes a noun and up to date when it follows a noun. E.g. Which is the most up-to-date version?, The manual is completely up to date.