If you don't want to install a mod menu, you can replace your local save files with a "100% Completion" save file.
There are three primary ways to achieve a full unlock on the PC (Steam) version of the game. Each varies in difficulty and risk. Using Mega Hack (v7 or v8) geometry dash all icons unlocked download pc work
Some players prefer "visual" unlocks. Texture packs don't technically unlock the icons in the game's logic, but they replace the default "easy-to-get" icons with the high-tier "hard-to-get" icons. If you don't want to install a mod