Substitute

Substitute[ exprorg, expr1, expr2 ]

Substitute[ exprorg, expr1, expr2 ] substitutes all occurrences of expression expr1 with expression expr2 in a given expression exprorg .

Example in out reduced
The expression x^4 + x^2+ 3x^2 + 6 with all occurrences of x^2 replaced by p. Substitute[x^4 + x^2+ 3x^2 + 6, x^2, p ] x^4 + p + 3p + 6 x^4 + 4p + 6