Do you mean you have a variable containing a numerical value, and you want to convert that into a text format for display?
eg, convert from number 63₁₀ = 0x3F = 111111₂ to text "3F" (ie, character '3' followed by character 'F').

Or are you receiving text-based input, and you want to convert that to a number for use in calculations?
eg, receive text "3F" (ie, character '3' followed by character 'F') and convert to number 63₁₀ = 0x3F = 111111₂

In either case, it's been discussed plenty before - try a 'Search'

Remember: when the value is stored as a number in a variable, its radix (hex, binary, decimal, etc) is irrelevant - you just use it as a number.